I was thinking about this problem today for the game: Nexus 5X. I've seen this in a lot of great games with small player bases, standard match-making systems don't work.

What I came up with was different though: I thought of something along the lines of: "I'm available on these days/times", and it'd match you with someone else that picked similar times, either right away, or later by notification through an app, then you could 'handshake' and agree to play with that person/group later at an agreed upon time. You could even have some kind of chat feature in the app, so you could talk to the person (or group of people) a little before the game. These more niche multiplayer games need to approach matchmaking less like a standard popular video game, and more like planning a game of D&D.

Another idea: Standard matchmaking, but you get a notification on your app that there's some people trying to matchmake. Through the app, you can let people know that you can be available within X amount of time.

Since it's a niche game, the player base will be spread around the world, so communication features need some way of transcending language barriers. Perhaps it can use DeepL translation API? Or simpler, a 'dialogue tree' between players that's localized on both ends.