To the transmission (and clutch) specialists

Hi! I have a bigger problem with my powertrain. I bought my E36 325i in the spring to convert it as a ring tool. At that time some gears had to slip in “problems”. I did everything completely new on the car anyway, so also a new clutch and a replacement gearbox (which I still had) built in. Unfortunately, the clutch seemed to me far too easy and was somehow spongy (I still have 2 more E36 for comparison, all 325i). new coupling hose pure and aired a thousand times, but no improvement. (By the way, the hose expands when operated, that was what the old like the new one did, what is it supposed to be??? Apparently normal, because the other E36 also do that..) But now I (and two car-meisters) think that the coupling separates in any case. But now also in the second transmission the first gear from idling and the second gear is switched off from the third. But just not always, that’s why you can’t say anything exact again. ICh can switch from the 5th to the 2nd, except with (real) intermediate gas, which would point to the synchronous rings again. I’m pretty perplexed by now… Who can give me another tip? Greeting