Looks like you have tried pretty much everything. Ive been having similar issues. Browsing is slow but if i run a bandwith test from anywhere its good. To add onto that, most of the time i am getting < 1mbps and downloading from play store is impossible.
Figure Verizon was throttling me even though i have unlimited + pay > $500 and dont really use much data cause 1mbps makea it hard.
I have seen 5g phones on Verizons 4g towers and they dont have issues.
If you havent tried, boot into safemode on your phone. It fixes my data issues but im not sure why yet. Had issue even after factory reset. If it works for you, atleast its a starting point.
Happens to be the reason i am searching these forums.