I don't think there is a way to do it (except if you mess around the code somehow and upload your version), the developer is very adamant about the whole 2 wave respawn time. It's disappointing.

Here is a link to a discussion about it:
https://steamhost.cn/steamcommunity_com/workshop/filedetails/discussion/1390992668/1741101364293698541/

Another forum thread about it:
https://steamhost.cn/steamcommunity_com/workshop/filedetails/discussion/1390992668/3397295779073026831/

The long respawn times are an unfortunate side effect of the choices we made when designing the mod. You're right to say that waiting for 20 minutes is not fun at all. We have tried to tackle this by making the waves shorter, but it's still a significant amount of time.

I can think of three hypothetical alternatives to the 2 wave respawn:
1. Change the system to respawn everyone at the end of every wave (1 wave respawn).
2. Change the system so that players respawn based on a timer.
3. Change the system so that players have a limited amount of lives, and they respawn shortly after dying.

A limited amount of lives actively discourages more risky playstyles, which we don't want. We want players to try all kinds of playstyles in the mod, so that's not a good option.

A timer based respawn could work, but might encourage players to keep one bot alive, and just wait for everyone to respawn before moving on in the invasion. To make this work, we'd probably have to change the way bots spawn to be much more continuous, rather than in waves.

Changing to a respawn at the end of every wave is the easiest alternative, but also brings a bunch of other consequences. For one, we've currently balanced the invasions so that only the strongest waves are capable of fully wiping out a large team (the Conduit wave in Blackmarch, for example). Generally speaking, we've balanced the invasions in such a way that the difficulty lies in surviving two consecutive waves. Changing to a single respawn wave would mean that (alomst) every wave should be balanced in such a way that it can fully beat a larger team. This would mean that bots get stronger, so players would die more often. If we don't do that, it'd become more or less impossible to lose.

This is why we choose to stick to the 2 wave respawn system: it's flawed because it makes players wait a long time, but it also means bots aren't invincible killing machines.

What about a limited amount of lives that increases every 3 or so waves, with a cap and no respawning mid-wave so you can't stockpile them for the final waves? Or recruits get to respawn the next wave, but soldiers stay dead for 2 to add a bit more incentive to not playing soldier?