When trying to play workshop maps in coop, the game often freezes when trying to load the first map, saying 'Downloading test chamber...', even if both of us already has the map in the queue. It usually takes 15-20 minutes of trial and error to load up the first map, but I feel that it's getting worse for some reason (more and more tries are needed). If we manage to load the first map, gameplay goes smoothly after that.

Anyone experiencing this issue as well? Are there any hotfixes or workarounds?

This happens very often and it clearly is a steam / workshop issue.
When it happens to me and my mates we usually restart the game (both players have to do it).
And then just wait for it without further restarts - it should not take more than 3 minutes then.

Another way which seems to work quite well (sometimes) is to start a standard coop game (just load the lobby/hub)
and then disconnect and invite your partner for a custom map.

As far as I can tell, it gets even worse on sundays and really depends on steam server load.
Restarting steam can work sometimes, but it might also cause a 5 minute "delay" for the first map.

One tip : sometimes it is helpful to have both partners download the map.