Maps with OpenStreetMap instead of Google Maps?

Only Garmin can answer that question. Ask Garmin Support and you should also send it to Garmin as a suggestion: https://www.garmin.com/forms/ideas/

Garmin Connect Web works in a web browser on the phone or tablet.

I usually plan my routes in Strava. It will then sync my routes to Garmin Connect and my devices. There are other services that do the same like Komoot.

I am using a de-googled phone as well, and I post here my disappointment about garmin connect android app not being able to render activities maps. I know it can be accessed in GCW, but i find it very odd that a company which one of the main activities is cartography devices which already support the open street map project on its website, does not allow their users use osm on their favorite device.

It is probably easier for them to support and manage a primary map service and to use one that people are the most familiar with.  

However, I think it would be nice to be able to easily toggle or have a default map type.  OSM is updated better and has much more detailed trail systems!  Recommend following the 'ideas' request that Andy posted.

NickMN said:

OSM is updated better

Osm in itself is constantly updated, yes. But for GC / Explore, Garmin is not using data from osm directly. They created their own copy which they almost never update. Last update (at least in in my area) was somewhere in 2019 or earlier. Giant waste of potential, as the updated data is available for free, Garmin just decided not to use it :(. 

I submitted an "idea" through e7andy provided link. I hope they will take it into account. For anyone reading this and also disapointed of the lack of osm support, go submit the idea as well so it will have more chance to get to anything.