...yours is a cautionary tale, alright. Garmin nowadays is verrry hard to comprehend...it used to be wayy simple.
The OLD handheld Garmins, (and Magellans too) had serial
port out...but you have to make a cable yourself or pay big$ for it.... Plus modern laptops will need the usb->db9 com port adapter doohickey.
I can see now why the old Garmin12xls are still popular on ebay, but their internal memory batteries will be failing so that's iffy. (although they can be replaced by a tinker).I have one and it's still useful away from the boat.
..another maybe-not-so-great idea for you is an old cellphone. SOME
of these have a REAL gps as in N5 Nokias but there are more. Of course, They're only as rugged as,well,a cellphone! And- they can all ensnare you with more fiddly brain-twisting to get charts
or non-OEM maps on them, just because you will
hear it's possible...