It depends on your threat model (how much you want to share).
Any device would leak a bit of information, either when it pings the satellite or cellular network, and some devices even ping nearby devices.
go back to a “dumb” computer (maybe not such a bad thing!)
It is difficult to find anything "dumb" nowadays. If you go to the vintage route, make sure they have a common cable, and that you can export/import the files using some easy to read format like GPX. To avoid getting stuck in proprietary bullshit.
That is even more important if you like to plan your route on your PC and send to the device.
keep my Garmin devices but take them offline
This will leak less information than your mobile device. Depending on the device, you can find open-source software that can make your life a lot easier for transferring files and reading them.
use a phone-based app like CoMaps
Can be an easy route. If you have your phone on you all the time, you are already leaking some information anyway.
or possibly a wearable fitness tracker.
Some of them require a proprietary app that definitely will phone-home with your info.
You can try GadgetBridge to avoid that https://gadgetbridge.org/gadgets/ but the supported devices list is small.
Maybe even an all-in-one platform like Garmin, but where the company really puts privacy front-and-center
No, sadly. The best you can find are community made open-source apps alternatives trying to bridge the gap.
Garmin used to show their source code back in the day. https://developer.garmin.com/open-source/linux/