[Magisk] ianmacd:
The problem is that the 64 bit binaries get built and installed, but Magisk segfaults on boot:

12-07 11:34:17.021 4170 4170 I Magisk : * Creating /sbin overlay
--------- beginning of crash
12-07 11:34:17.021 4170 4170 F libc : Fatal signal 11 (SIGSEGV), code 2, fault addr 0x601b7eb050 in tid 4170 (magisk)
12-07 11:34:17.041 4173 4173 I crash_dump64: obtaining output fd from tombstoned
12-07 11:34:17.041 4173 4173 E libc : failed to connect to tombstoned: No such file or directory
12-07 11:34:17.042 4173 4173 I crash_dump64: performing dump of process 4170 (target tid = 4170)
12-07 11:34:17.042 4173 4173 F DEBUG : *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
12-07 11:34:17.042 4173 4173 F DEBUG : Build fingerprint: 'samsung/star2ltexx/star2lte:8.0.0/R16NW/G965FXXU2BRJ8:user/release-keys'
12-07 11:34:17.042 4173 4173 F DEBUG : Revision: '26'
12-07 11:34:17.042 4173 4173 F DEBUG : ABI: 'arm64'
12-07 11:34:17.042 4173 4173 F DEBUG : pid: 4170, tid: 4170, name: magisk >>> magisk <<<
12-07 11:34:17.042 4173 4173 F DEBUG : signal 11 (SIGSEGV), code 2 (SEGV_ACCERR), fault addr 0x601b7eb050
12-07 11:34:17.042 4173 4173 F DEBUG : x0 0000007a9ce4e100 x1 0000007a9ce4e115 x2 0000000000000005 x3 0000000000000010
12-07 11:34:17.042 4173 4173 F DEBUG : x4 0000000000000020 x5 0000000000000018 x6 0000007a9ce2fb15 x7 6f723a725f746365
12-07 11:34:17.042 4173 4173 F DEBUG : x8 0030733a7366746f x9 18abf24110985161 x10 0000000000000004 x11 0000007a9ce0d660
12-07 11:34:17.042 4173 4173 F DEBUG : x12 000000000000001f x13 0000000000000000 x14 ff00000000000000 x15 3227282401000000
12-07 11:34:17.042 4173 4173 F DEBUG : x16 0000007a9e0a1cf0 x17 000000601b7eb050 x18 0000000000004220 x19 0000007ff0fa8d00
12-07 11:34:17.042 4173 4173 F DEBUG : x20 0000000000000000 x21 0000007a9ce4e100 x22 0000000000000001 x23 0000000000000000
12-07 11:34:17.042 4173 4173 F DEBUG : x24 000000601b7d1d98 x25 0000000000000000 x26 0000000000000000 x27 0000007a9e45da60
12-07 11:34:17.042 4173 4173 F DEBUG : x28 0000000000000000 x29 0000007ff0fa8cf0 x30 0000007a9e095e3c
12-07 11:34:17.042 4173 4173 F DEBUG : sp 0000007ff0fa8cc0 pc 000000601b7eb050 pstate 0000000020000000
12-07 11:34:17.044 4173 4173 F DEBUG :
12-07 11:34:17.044 4173 4173 F DEBUG : backtrace:
12-07 11:34:17.045 4173 4173 F DEBUG : #00 pc 0000000000000050 /sbin/magisk (offset 0x20000)
12-07 11:34:17.045 4173 4173 F DEBUG : #01 pc 0000000000011e38 /system/lib64/libselinux.so (lgetfilecon+88)
12-07 11:34:17.045 4173 4173 F DEBUG : #02 pc 0000000000014334 /sbin/magisk
12-07 11:34:17.045 4173 4173 F DEBUG : #03 pc 0000000000014a04 /sbin/magisk
12-07 11:34:17.045 4173 4173 F DEBUG : #04 pc 0000000000007a8c /sbin/magisk
12-07 11:34:17.045 4173 4173 F DEBUG : #05 pc 0000000000005da8 /sbin/magisk
12-07 11:34:17.045 4173 4173 F DEBUG : #06 pc 0000000000004e6c /sbin/magisk
12-07 11:34:17.045 4173 4173 F DEBUG : #07 pc 0000000000004d5c /sbin/magisk (main+152)
12-07 11:34:17.045 4173 4173 F DEBUG : #08 pc 000000000001b94c /system/lib64/libc.so (__libc_init+92)
12-07 11:34:17.045 4173 4173 F DEBUG : #09 pc 0000000000004ca8 /sbin/magisk
12-07 11:34:17.050 4173 4173 E crash_dump64: unable to connect to activity manager: Connection refused

I haven't yet been able to figure out why this is happening.

That might work; I don't know. It's obviously not ideal to do it that way, but you could try.