[Google Pixel 4a ROMs, Kernels, Recoveries, & Other] DiamondJohn:
So much to cover. It really looks like a "Death by a thousand cuts". No individual killers, but the sum of all the steps. Each step a potential point for failure.

There is an old principal, which I work by. ====> K.I.S.S.

KEEP​

IT​

SIMPLE​

STUPID​

Things I read that just sounded destined to add a possible failure point, and hence raising the probability of failure. No ONE specific Gotch-ya, and each individual one may of worked for someone. But, put them all together and...

So my suggestions:

Download and use the LATEST FASTBOOT. I use fastboot version 34.0.3-10161052 It has a KNOWN bug, but I read that as the previous versions have multiple unknown bugs (see below). They upgraded for a reason.

Do NOT use powershell

Do NOT start the command shell, via powershell. Every layer adds a layer of complexity, which adds a possible failure point. Start the command shell directly from the start bar.

Do not use long paths, and keep the path name to less than 8 characters. Preferably directly off a root (see below)

Do NOT use paths with spaces in them. I consider that a big one

Make certain you are running the correct stock firmware. I have forgotten which it is, I am guessing I have put it somewhere in the thread. If I haven't, then chose the month BEFORE I built the ROM. And I guess the bootloader version is in the screen-out below.

For me, I created and copied the install zip's to a folder F:\risingos (I have a LOT of drives). I downloaded and unzipped the ADB/Fastboot tools to the folder "C:\Program Files\Java\Android\platform-tools"Spoiler content hidden.

I then worked around the bug in the latest fastboot, with the line

set ANDROID_PRODUCT_OUT=f:\risingos

The below is basically my command shell commands & output when I flashed it. Successfully. Spoiler content hidden.

Microsoft Windows [Version 10.0.22621.1702]
(c) Microsoft Corporation. All rights reserved.

C:\Program Files\Java\Android\platform-tools>f:

f:\>cd flashros

f:\flashros>PATH=%PATH%;"C:\Program Files\Java\Android\platform-tools"
f:\flashros>fastboot --version
fastboot version 34.0.3-10161052
Installed as C:\Program Files\Java\Android\platform-tools\fastboot.exe

f:\flashros>set ANDROID_PRODUCT_OUT=f:\risingos

f:\flashros>fastboot devices
xxxxxxxxxxxxxx fastboot

f:\flashros>fastboot update -w risingOS-v1.1-Babylon-202306021421-sunfish-CORE-COMMUNITY-updateimages.zip
--------------------------------------------
Bootloader Version...: s5-0.5-9430386
Baseband Version.....: g7150-00108-230117-B-9497111
Serial Number........: XXXXXXXXXXXXXXXX
--------------------------------------------
extracting android-info.txt (0 MB) to RAM...
Checking 'product' OKAY [ 0.063s]
Setting current slot to 'b' OKAY [ 0.155s]
extracting boot.img (64 MB) to disk... took 1.259s
archive does not contain 'boot.sig'
Sending 'boot_b' (65536 KB) OKAY [ 1.649s]
Writing 'boot_b' OKAY [ 0.310s]
archive does not contain 'init_boot.img'
extracting dtbo.img (8 MB) to disk... took 0.093s
archive does not contain 'dtbo.sig'
Sending 'dtbo_b' (8192 KB) OKAY [ 0.310s]
Writing 'dtbo_b' OKAY [ 0.094s]
archive does not contain 'dt.img'
archive does not contain 'pvmfw.img'
archive does not contain 'recovery.img'
extracting vbmeta.img (0 MB) to disk... took 0.001s
archive does not contain 'vbmeta.sig'
Sending 'vbmeta_b' (4 KB) OKAY [ 0.137s]
Writing 'vbmeta_b' OKAY [ 0.078s]
extracting vbmeta_system.img (0 MB) to disk... took 0.000s
archive does not contain 'vbmeta_system.sig'
Sending 'vbmeta_system_b' (4 KB) OKAY [ 0.140s]
Writing 'vbmeta_system_b' OKAY [ 0.078s]
archive does not contain 'vbmeta_vendor.img'
archive does not contain 'vendor_boot.img'
archive does not contain 'vendor_kernel_boot.img'
extracting super_empty.img (0 MB) to disk... took 0.001s
Rebooting into fastboot OKAY [ 0.070s]
< waiting for any device >
Sending 'super' (4 KB) OKAY [ 0.001s]
Updating super partition OKAY [ 1.027s]
Resizing 'product_b' OKAY [ 0.007s]
Resizing 'system_b' OKAY [ 0.007s]
Resizing 'system_ext_b' OKAY [ 0.007s]
Resizing 'system_a' OKAY [ 0.007s]
Resizing 'vendor_b' OKAY [ 0.007s]
Resizing 'vendor_a' OKAY [ 0.008s]
archive does not contain 'boot_other.img'
archive does not contain 'odm.img'
archive does not contain 'odm_dlkm.img'
extracting product.img (1652 MB) to disk... took 36.715s
archive does not contain 'product.sig'
Resizing 'product_b' OKAY [ 0.008s]
Sending sparse 'product_b' 1/7 (261913 KB) OKAY [ 12.520s]
Writing 'product_b' OKAY [ 1.905s]
Sending sparse 'product_b' 2/7 (261702 KB) OKAY [ 7.656s]
Writing 'product_b' OKAY [ 0.975s]
Sending sparse 'product_b' 3/7 (262012 KB) OKAY [ 10.855s]
Writing 'product_b' OKAY [ 0.852s]
Sending sparse 'product_b' 4/7 (261583 KB) OKAY [ 7.696s]
Writing 'product_b' OKAY [ 1.048s]
Sending sparse 'product_b' 5/7 (261076 KB) OKAY [ 7.252s]
Writing 'product_b' OKAY [ 1.145s]
Sending sparse 'product_b' 6/7 (262140 KB) OKAY [ 9.498s]
Writing 'product_b' OKAY [ 0.835s]
Sending sparse 'product_b' 7/7 (108755 KB) OKAY [ 3.677s]
Writing 'product_b' OKAY [ 1.148s]
extracting system.img (868 MB) to disk... took 16.371s
archive does not contain 'system.sig'
Resizing 'system_b' OKAY [ 0.009s]
Sending sparse 'system_b' 1/4 (262092 KB) OKAY [ 6.290s]
Writing 'system_b' OKAY [ 1.876s]
Sending sparse 'system_b' 2/4 (262104 KB) OKAY [ 6.271s]
Writing 'system_b' OKAY [ 0.819s]
Sending sparse 'system_b' 3/4 (262004 KB) OKAY [ 6.321s]
Writing 'system_b' OKAY [ 0.905s]
Sending sparse 'system_b' 4/4 (99572 KB) OKAY [ 2.384s]
Writing 'system_b' OKAY [ 0.360s]
archive does not contain 'system_dlkm.img'
extracting system_ext.img (512 MB) to disk... took 9.530s
archive does not contain 'system_ext.sig'
Resizing 'system_ext_b' OKAY [ 0.009s]
Sending sparse 'system_ext_b' 1/2 (262032 KB) OKAY [ 6.284s]
Writing 'system_ext_b' OKAY [ 1.909s]
Sending sparse 'system_ext_b' 2/2 (253887 KB) OKAY [ 6.182s]
Writing 'system_ext_b' OKAY [ 1.883s]
extracting system_other.img (11 MB) to disk... took 0.590s
archive does not contain 'system.sig'
Resizing 'system_a' OKAY [ 0.007s]
Sending 'system_a' (12216 KB) OKAY [ 0.290s]
Writing 'system_a' OKAY [ 0.101s]
extracting vendor.img (637 MB) to disk... took 11.806s
archive does not contain 'vendor.sig'
Resizing 'vendor_b' OKAY [ 0.007s]
Sending sparse 'vendor_b' 1/3 (259776 KB) OKAY [ 6.248s]
Writing 'vendor_b' OKAY [ 1.907s]
Sending sparse 'vendor_b' 2/3 (262140 KB) OKAY [ 6.317s]
Writing 'vendor_b' OKAY [ 0.823s]
Sending sparse 'vendor_b' 3/3 (126476 KB) OKAY [ 3.058s]
Writing 'vendor_b' OKAY [ 0.474s]
archive does not contain 'vendor_dlkm.img'
archive does not contain 'vendor_other.img'
Erasing 'userdata' OKAY [ 8.856s]
Erase successful, but not automatically formatting.
File system type raw not supported.
Erasing 'metadata' OKAY [ 0.014s]
Erase successful, but not automatically formatting.
File system type raw not supported.
Rebooting OKAY [ 0.001s]
Finished. Total time: 247.694s

f:\flashros>fastboot devices
xxxxxxxxxxxxxx fastboot

f:\flashros>fastboot flash --slot all boot boot1_magisk_patched-26100_a0SJk.img
Sending 'boot_a' (65536 KB) OKAY [ 1.700s]
Writing 'boot_a' OKAY [ 0.311s]
Sending 'boot_b' (65536 KB) OKAY [ 1.680s]
Writing 'boot_b' OKAY [ 0.361s]
Finished. Total time: 4.750s

f:\flashros>

If anyone has issues or would ask for help, I really don't see how anyone could expect assistance without similar output I gave above. There is so much info above that people are not talking about. eg bootloader version.