iserith
Magic Wordsmith
My idea was to replace the Advantage granted from flanking with just a +1 to attacks against a flanked opponent.
I'm not sure that really succeeds at achieving the goal you set forth. It makes the game more fiddly and prone to people trying reversies when they inevitably forget they didn't apply the bonus they should have. It risks creating problems you might not have now for no real upside.
I would suggest that if you want the combats to be more tactical, introduce better terrain and alternate victory conditions. If you want to do damage to highly armored PCs, use spells with saving throws against them. If you want multiple weaker monsters to serve as a threat, look at the Mob rules in the DMG, page 250, which are basically auto-hits given a certain number of attacks (at the cost of never having any crits).