Mongo has a lot of performance gotchas with things like pipelines / aggregations that don’t scale the same way they do in Postgres & similar
Took a while but we eventually migrated to a plain rest api with a react front end. Still used mongo
Meteor was fantastic for building highly reactive apps with a small team. I think it got a lot of bad run for scaling up because it was dependent (at least when I used it) on tailing MongoDB's op log for reactivity. That lead to having to make very different architecture choices to make it work well, and I seem to recall each server instance getting bogged when they had 70-100 concurrent users , so you ended up using lots of containers for meteor application servers. Not sure where it all ended because I moved on to a different platform three yeas ago.