[Samsung Galaxy S20 / S20+ / S20 Ultra Questions & ] sjevtic:
First off, why do you even want Android 12 so badly? Early adopters catch all the bugs. And moreover, if you want to mod your phone, you'll quickly notice that a lot of popular mods aren't yet available for Android 12. As of today:

There still is not an official Magisk release for Android 12 (only Canary/pre-release builds).

Xposed module GravityBox for Android 12 has not been released.

Xposed module Firefds Kit for Android 12 has not been released.

And if you start modding, you're likely to find more things that just don't exist for A12 yet.

Yes.

It used to be fairly easy. You could open the Preconfig dialog with a dialer code and change your CSC code. Loss of this functionality in recent software is lamented loss of this functionality in recent software is lamented here. There are many discussions about changing CSC code that make mention of the commercial software and service Samkey such as this and this; I have no personal experience with their offering so I can't recommend them. Interestingly it seems from the discussions that there is typically some lag between an Android release and support for that release from Samkey, so the trend I just referenced above is hardly an isolated phenomenon.

It's worth mentioning that the changing of CSC codes has been discussed extensively throughout the forum. One of the downsides of XDA though is that discussion is heavily segregated by device, so you can easily miss what you're looking for if you search only in a sub-forum for your device.

With root, certainly. Otherwise, it is not so clear based on the above.

You can launch the Preconfig dialog from a root shell:

am start -n com.samsung.android.cidmanager/.preconfig.PreconfigActivity

I've noticed that in some configurations (notably when CSC code is VZW), the Preconfig dialog seems to be somehow disabled. However, with root access you can also simply edit /efs/imei/omcnw_code.dat.

That's your choice. Browse the ROM section and see if there is anything available for your device that you like. Custom ROMs vary greatly in user experience as a consequence both the ROM's objectives and the developer's skill.

They all have a few things in common though: they will irrevocably trip the warranty void bit and cause a loss of Knox-related functionality (notably including Samsung Pay). They also will prevent you from receiving official OTA updates.

Another option is that you can simply root your device and start modding away yourself. Basically, that workflow is something like this:

OEM Unlock.

Boot into download mode with vol up + vol down + insert USB cable connected to PC.

Volume up to confirm unlock.

Flash TWRP from ODIN.

Flash Multi-disabler from TWRP.

Wipe /data from TWRP.

Install Magisk from TWRP.

Once you have root, you can not only change your CSC code as desired, but edit the read-only filesystems on your device, notably including the filesystems containing the CSC. It's even possible to blow away the CSC content completely and integrate the CSC content from the ROM of another region.

Of course, it goes without saying that you need to back up anything important on your phone before you start any experiments. Of particular importance, be sure to back up efs and sec_efs with TWRP and make sure the backups are stored somewhere safe (e.g., on a PC) before you start attempting any changes to efs partitions. A damaged efs partition is one of the easiest ways to render your phone useless. Note also that custom ROM installation almost always involves wiping /data somewhere along the way.