I have a home in Ocean City, and spend most of my summers there as well as holidays year around...I’ll try to help here, but I’d be happy to answer any questions you may have if you want to message me too.
I wouldn’t recommend staying on the lower part (below about 12th Street) because of noise. You can find some ok places to stay in the lower blocks, but many will be slightly run down and filled with college/high school kids. There are some nicer, newer hotels on the bayside that are great for sunsets - like the Hampton Inn. If you don’t mind driving about five minutes to the beach, the area of West Ocean City (the same area as the Francis Scott Key Resort) has several very affordable hotel options. Many of them quite new.
Also, parking down by the boardwalk is not a big deal - there is a huge lot at the inlet where you can park for $3.00/hour. I’ve seen it busy, but never full. There are also many side parking lots, but the inlet is easiest to find and will put you right down in the midst of the games/rides/arcades/shops on the busiest part of the Boards.
There is a crazy amount of things to do in OC - mini golf, amusement parks, water parks, zip lining, go karts, boats, jet skis, parasailing, countless restaurants....if you’re in need of specific recommendations, let me know.
Assateague is beautiful, but not much different than OC beach-wise. If you want to go see the horses, then yes, it’s worthwhile. You will need to pay if you want to go into the National Seashore side of the island, but it’s approx. $7 for a week long pass. You may just be able to go over the bridge and see horses without ever reaching the pay booth. It’s about a 20 minute drive from the Route 50 bridge to Assateague.
Hope that helps. Enjoy your trip.