It sounds like your almost describing the Nym mixnet
https://youtu.be/_2DQ_iYZi5U?t=1580
The tradeoff is you necessarily need to smooth traffic bursts out to meet the fixed rate and that introduces high latency. Unfortunately most user traffic is bursty and not continuous.
It would actually be pretty easy to implement for tor (either for the whole network, or individual nodes or routes), but as far as I can see nobody wants to work on it.