I grew up driving a manual transmission. My first driving test was in a 1958 ford pickup. My first car was a 1950 ford coupe. Both manual transmissions. In fact I didn't own an automatic transmission vehicle until the late 90's. I used to take my 1978 Toyota Celica fastback out on to the deserts of western Utah and just beat the crap out of that thing racing around on the desert. I only ever blew the clutch once. and never a transmission. So while I never drove an actual race car, I do have some experience with manual trannies. Quite honestly there is not a racing sim on the market that has a realistic manual clutch and trannie. And yes s3e comes the closest. And I understand that that is probably one of the harder parts of a car to reproduce in a racing sim. But could you please at least beef them up a tiny bit. Not so fragile on the down shift. Or use the bing noise to tell drivers that the sincro's will not let the transmission go into gear. Like in real life. If you are not within a certain performance ratio the car wont go into a lower gear. Instead of trashing the trans and engine. My favorites are the legends partly because they are closer to my "glory days" as it were. and the use of the clutch just feels that much closer to reality to me. I've been using the automatic clutch. But that just doesn't feel right to me in more ways than I care to mention here. And the manual clutch isn't there either. Just politely asking. I love most everything else about the game. 5 stars to say the least.
We've got our work cut out for us for quite a while ahead but there's always a little bit of time to at least investigate things like this. I don't want to promise anything but I can poke the devs to see if there's something we can improve (that won't take forever and give minimal results).
The real question here is, though - do you have experience driving a car with unsychronized transmission? Because it kinda sounds like you don't. I'd expect even your 1950 Ford had a synchronized transmission, let alone the newer cars you mention. Though I do think transmission might be a bit too fragile in R3E, you really can't expect to be able to drive with an unsynchronized transmission the same way you'd drive with a synchronized transmission. Especially when it comes to downshifts.