Does this use TCP over TCP (painful in the face of packet loss[1]) or can you do something like using QUIC for the forward proxy to try to avoid breaking the tunneled TLS connection's retry timers?
It looks like a normal HTTP proxy supporting CONNECT (i.e. TLS over TLS), which wouldn't suffer from the problem you mention.
Note that TLS over TLS is _not_ the same thing as TCP over TCP. TCP over TCP is usually only a problem for VPNs or something similar (i.e. anything that sends raw IP packets over TCP).
Http3 support is being talked about in an issue (am mobile so no link for you right now) but the first priority -- pending dev resources -- is to merge the v2 PR and vet for bugs.
[1]: http://sites.inka.de/sites/bigred/devel/tcp-tcp.html