100% agree. This game has been going on for about 3 years. Something like 6 months or so after we started, people began to catch on that adv flanking was not a good rule, but the GMs of the game are weirdly opposed to doing anything about it. They all say it's a problem, but they won't change it. And thus 3 years goes by (why the heck I decided to play a barbarian in such a game, idk. Reckless and grappling are far less valuable when simple flanking does 85% of that work).
While in theory I like adding proficiency bonus, having a flat value has simplicity merits. +3 proficiency is where the bulk of playtime happens IME, so my favored solution is to say flanking provides a +3 bonus.