I'd suggest the devs look at the slavery mechanic in Civ4, In Humankind it definitely does not really feel like a trade-off at present. 

I liked another user's (Krikkitone) suggestion of linking a stability malus to prevent population buy-out spamming.

I think money buy-out is alright without limitations, so long as gold income and buy-out costs are balanced well.

Indeed in Endless Legend districts were bound to pop count

That game also had resoruce extraction districts which didnt require any pops

Since its the same company there must be a design reason why they lifted the pop limitation to districts - it might still be valid

But as is both districts and infra seem OP.

Districts have limitation to them via stability but infra doesnt - the reasoning here is likely that infra buildings can only be built one per city while districts are largely unlimited in their number

Having something much more restrictive as a limiting factor for both would make balancing easier but maybe less fun for players

I would probably try to limit districts by pop to represent the growing cities better and prevent district buyout spam - organic growth

And limit infra via Stability instead and make it a much more harsh penalty so that each city can only have only 2-3 additional infra per default per era

As we can spam both districts and infra via buyouts of various kinds, by ranking up production in the begining or via both

In my last game I didnt need to build a single infra building, districts wre more then enough

An alternative approach might be to give stabiltiy increase to infa buildigns and districts having mcuh higher cost in stability

The idea here would be that only 1-2 districts can be built per era per city due to the amount of new infra buildings made available in that era

This would be a very simple balancing mechanic and makes sense in a way to improve infra before being able to expand the city physically

The downside of this solution is that it would still not limit infra per city

Ideally the player would chose both infra and districts as per geography and city function they are aiming at. While this is somewhat true for infra, districts can be placced very easily and thankds to adjecency bonuses they rank up fast. Adjecancy bonus, buyout options and no maintenance all need to be balanced by a strong concept