So I got my proper replacement (not a repair this time) about 2 weeks ago now. I didn't want to say much just in case this one still freezes & restarts. Everything that was freezing the previous ones isn't freezing this one, I have been using it heavily. Not even any bad magic crashes (or whatever crash log replaced that in 10.15.4).
The macbook arrived with 10.15.3 pre-installed, 10.15.4 was released while it was shipping over to me. The build date is March 2020.
With 10.15.3 pre-installed, it froze and restarted with no 3rd party software. Did initial setup, logged in and it froze while downloading files. I then saw 10.15.4 had been released and updated through system prefences, over the top of 10.15.3. It froze and restarted in sleep, right after system update, nothing installed.
I thought stuff it, I'll try a clean install, nothing was on there anyway. After the clean install it hasn't frozen for 2 weeks, with about 200 apps installed for work and doing a lot of heavy tasks on it. Peripherals plugged in, external monitors. No crash, all good.
For the clean install (and I think this is important), I used Internet Recovery (hold command + R on boot). Before that though, I reset the NVRAM & SMC, then once those were done I entered Internet Recovery. In Internet Recovery, before installing 10.15.4 I went in to Disk Utility and deleted both APFS containers (right click on drive and choose to delete each container) for the main internal drive, then erased the disk after deleting containers. Apple support never tell you to erase the APFS containers. The T2 encrypts the APFS containers and I think this is where the issue stems from. After deleting the containers & erasing, I installed 10.15.4. Since doing that, the macbook hasn't crashed once, no issues whatsoever for a solid 2 weeks with a lot of use every day. I didn't restore any backups whatsoever, I spent a few days downloading apps fresh from the web & copied data in user folders manually over from backup drive.
Who actually knows if it's fixed since apple won't admit a thing in regards to this situation, not even make note of it in update changelogs. Take this advice with a grain of salt, don't hold me responsible for erasing your system and the problem is still there...this is just my experience.
Either way this is what I expected "off the shelf" and I've lost a lot of time and even some clients to this situation. 5 months and I finally seem to have a mbp 2019 16" that works. 10.15.1 -> 10.15.3 appear to be dudds with this machine but 10.15.4 is working for me, after a totally clean install.
I hope this helps, I was going to wait a month before reporting but it pains me to see so many still affected by this. Unsure why it doesn't pain apple engineers the same way. My original mbp 2019 16" and logicboard replacements had November 2019 build dates, this latest one is a March 2020 build date, so this might not work on 'older' builds.