If you can hide an old small android phone with a a SIM card in your bike, you can use FindMyDevice (FMD) available in f-droid.

If not you can use some pet tracking device, like FindMyCat. It’s a small opensource device in a collar where you need to plug in a SIM card.

But if you want a device which uses low power Bluetooth / mesh connection and which doesn’t require SIM card nor GPS. Then I don’t know any alternative which is not a privacy nightmare.

I use Logseq with ToDoList plugin to handle task list. Logseq is similar to obsidian but open-source. And to syncronize notes among devices, I use syncthing. No need to selfhost.