Recently I have pushed an update to my tool called
Eternal Witness (
https://gitlab.com/Dragas/eternal-witness), which permits users to host events with their own deckbuilding rules and modifiers. The (only*) caveat right now with it is you need to trust your players to run the deck they had submitted.
To encourage people to use the feature, I am hosting an event called Ethiopian BIRRRR. It's a modern-based event, where 6 cards from modern pool are unbanned, one card is banned, and some additional deckbuilding restrictions are imposed. Check it out on
https://traditionalmtg.github.io/birr.html.
Currently the tool supports only discord as interface, but I am looking into migrating it to a real website. If you would like to host your own events with your own rulesets (30 card singleton, for example), let me know and I'll set you up.