For PoS, issuance is a formula depending on the amount staked. How that works out: https://docs.ethhub.io/ethereum-roadmap/ethereum-2.0/eth-2.0...
If you want to dig into the actual formula, it's explained here: https://benjaminion.xyz/eth2-annotated-spec/phase0/beacon-ch...
Until PoW migrates to PoS, probably in about a year, they'll operate in parallel and we'll have both rewards added together.
A final factor is an upcoming change to the way transaction fees work, which will burn most of the fees instead of awarding them to validators. That will reduce net issuance further, possibly even taking it negative.
In any case, by the time the full transition to PoS is complete, one thing that will be easy for the market to know is that issuance will be substantially lower than BTC, and possibly negative depending on transaction fees.
As a matter of fact, knowing how many ETH will be produced over a given period of time is a question I never managed to get a satisfactory answer to.
There's probably an algorithm buried somewhere in the code, but it's likely not a simple one.