I am sorry to be a bearer of bad news, but adding right slide release on Cz-75B is not a simple or inexpensive job either. You have explained what you are trying to accomplish. Your best bet is to find 85B. They are in existence, you just need to be patient. Keep checking Gunbroker. There is one 85B now, although a bit expensive: https://www.gunbroker.com/item/990019438 IMHO 85 Combat would be much better, but you insist on having firing pin block.
Had to pass on that one: starting bid of 900 (plus all the fees) for a used gun that was completely stock and nearly a quarter of a century old. You're right though, it looks like it'll just be a waiting game or lucking out with a random jackpot find in a LGS (which I'm not holding my breath for obviously)