It is some what of a mess at the moment.
Laz/fpc/Ultibo run on Rasbian Buster 32, that works now.
The "best" 64bit OS at the moment is Gentoo64 but that requires aarch64 version of the fpc compiler.
You can get the 3.3.1 fpc aarch64 binary version, it works
I have yet to figure out how to build a 64bit Laz from fpc 3.3.1.
The Laz/fpc/Utibo make scripts are using the older arm 32bit compilers.
Lazarus Version 2.0/fpc 3.04/Ultibo use fpc 3.02 to build themselves.
But aarch64 bit support is from 3.2 onwards.
Once you have a Laz/fpc/Ultibo running on a 64bit kernel then you need a cross compiler back to 32bit armhf.
I imagine once Laz/FPC move the build scripts to 3.2 or 3.3, things will get easier.
Aarch64 support is much better in 3.2+.
We just need to wait.
But for Pi4 support USB/Ethernet/VC6 etc need to be redone.
In two versions? 32 and 64bit?
More waiting, this time for the Ultibo team and probably the 2711 manual?
I don't think we will see the Zero disappear any time soon, it is too cost effective for some stuff.
As long as Broadcom still make the BCM2835 it will probably be around.