That’s odd. I hate closed eco systems.

If proton supports CalDAV (I’m not sure), it should work e.g. with DAVx5 which integrates well with Android calendar.

When I had the issue with mariadb demon been killed, I think either in dmesg or syslog there was an entry reading "Out of memory: Kill process… " or similar.

In principle any DAV client should work. On Android, I use DAVx5 and Tasks.org to access my Nextcloud. On Thunderbird + Lightning, you can e.g. use SOGo connector or TbSync to use the address books, calendars and tasks.

Then, maybe, Baikal a Cal-/ Card-DAV server is what you are searching for.

That would not be equivalent to ‘remote location’, as the files would be stored locally on your computer and being synced with the cloud.

The title states:

Attempting to use Home Assistant OS

So OP uses the preconfigured “ready to go” solution with the supervisor doing the setup and upgrades. Which somehow is not succesfull in doing so.