I felt like the software “kanboard” fit the deal the most in terms
of lightweight but I feel like the tracking abilities are kinda
restrictive for my needs and Google Keep can do the most of it without
additional maintenance.
There's also Nextcloud Deck, which has an Android companion app. https://f-droid.org/en/packages/it.niedermann.nextcloud.deck/ I am not sure how well Deck and Notes inside a Nextcloud web interface could work together but if I remember correctly Deck entries show up as Tasks in Nextcloud, in your customized welcome dashboard after logging in.