I am also - and always will be - a joplin-user and thought about outsourcing the tasks/todos. Was looking for selfhostable, opensource alternative with a mobile app (iOS) but it was harder to find sth than i expected.
These were my candidates:
SiYuan -> selfhosting does not work with apps (web only)
Anytype -> did not get the server to run (there are a lot of variables in the dockercompose.yaml und .env)