Alternatives to Opportunity Attacks

I'm running a "low magic" campaign where magic users and creatures are rare and seen to be very powerful (which is represented nicely with a level 7 low magic party as even basic spells still hit really hard and don't have a lot of counters from martial classes).

However, I'm stuck in a position where most encounters are martial focussed (to leave room for special magic encounters relevant to the lore), which is fine for those looking for their martial power fantasy, and we're a descriptive group that loves RP and martial combat gives a lot more opportunity for character expression than hurling fireballs from across the map. But we run into the issue of almost entirely static combat. Absolutely nobody wants to move and everyone is locked into position, trading blows for the entire encounter.

We've been playing this campaign for years and I'm struggling to create enough unique objectives and environmental issues to force more dynamic combat. And some of them are having the adverse effect of causing the players to play more safely and move even less.

The main culprit at play here is the Opportunity Attack, made even more damaging by the constant use of Polearm Master, Grapples, and Sentry. So even when I make enemies disengage and back away, or even just take the hit in order to reposition (for no real benefit at all other than to break up the "front line") they get prevented and are forced to stay put and it drags combat out and causes players to ignore or avoid half the features of the encounter I spent hours writing (I've reused unused elements several times in encounters before they've been triggered).

Players don't have enough reason to reposition either, and they don't have the option to retreat even when on low health since Disengage as a primary action feels like a complete waste of a turn.

So my question is; what do other DMs do in this position?

We have a heavily homebrewed campaign already, so the ideas of rules changes or completely new mechanics are welcome.

I don't want to remove OA altogether as players have some builds created around it and have been using them for years. I also want to avoid the skirmish style combat where players essentially abuse the nature of turn-based combat and run around an enemy immobilised by the turn order.

(I apologise for the long intro, but I wanted to avoid comments like "just use teleport" as it simply wouldn't work in our setting).