I get the appeal of a self-hosted tool, but my suggestion is to do the important ones in person.

When I ran a Linux group years ago, the decisions that mattered were settled around a table. For a big decision, the dialogue and the discussion that lead to a common agreement are the real work, and in my experience no software does that part for you.

For small stuff, a simple poll (or no poll at all) is fine: the core team decides and shares it as information. Save people’s attention for the decisions that need it.

I know of an Android app - Urn. There's also a web version at https://app.mieuxvoter.fr/en - the code for the website is from the same organization but I'm not sure how straightforward it is for others to host.

framadate.org seems like an option. Created by the same people that made Peertube.

https://pollen.cl/

Made by a french company pushing for more libre software out there. They're the ones behind https://copiepublique.fr/en/

Utmost*, for the sake of accuracy.

Nextcloud Forms have a ranked choice question type. The result shows the admin a Borda count, though for some reason it's calculated on the front end, not on the server. Easy to set up, to share links, and to vote. Can be anonymous or require an account. Implementing Schulze STV or another multi-winner alternative is in my plans, but idk when I'll get to that.

Survey software is probably the simplest way to do this.