I think the southern part of Anne Arundel County would be a great fit for you assuming as you said, budget is not an issue (and housing is extremely limited, rentals even more so). PG County is just south, also not a bad choice, but much nicer housing and water access in AACO. It would be a straightforward commute to Navy Yard once a week, that is not an abnormal commute from the area even daily. For specific barn recommendations of places that would offer SJ and dressage lessons but not require you be in a full program, I would look at Dovecote Equestrian, Trevi Manor, Liberty Ridge Stables (PG County), and Glenwood Farm (this one is particularly difficult to get into). All are great options that prioritize horse care, time outside, offer lessons and frequently participate in clinics. As far as I know none of them currently require you to be in a program. All of these also have a great culture of adult amateurs that like to socialize and support each other at shows but also respect that everyone is capable of doing their own thing. For human housing I would look in the Annapolis, Edgewater, and Davidsonville areas, going as far as Harwood and West River if you prefer more rural.
The water access in Anne Arundel is amazing - you can paddleboard, kayak, sail, etc. and itās really all around you. The only thing that might be a deterrent is lack of 10 mile + running trails, but as someone mentioned, you can get to a few bigger parks within a 30 minute or so drive and really all of western MD is an easy day trip. You are also only 2 hours from the beaches of MD and DE! As someone who has always had to drive 30 mins to an hour to get the absolute best barn experience possible, I imagine your SO is willing to do the same for his hobby, but it might not be exactly what he is looking for in that regard.