Steinway L -- a new (black) one would go for more than 50,000 USD now, I think.
Piano is made mostly of wood, and wood reacts to relative humidity. Relative humidity is key (not temperature by itself). Wood swells when its humid, and shrinks when it's dry. It's the nature of wood. If the wood shrinks or swells too much, it
may cause various problems in the piano. Even if you cannot treat the whole room to stabilize the room's humidity, there's something called Dampp-Chaser (
https://www.dampp-chaser.com/ )that's designed to help stabilize the humidity in the micro-climate surrounding the piano. You can give that a look.
But seeing that you're having problems with your piano right now, the best course of action really is to get a professional, a competent piano technician, to take a look, assess the situation, and fix stuff and give longer-term maintenance advise accordingly. If anything, getting a technician to bring your piano back up to good playing condition should be much cheaper than buying a new piano. If you learn from the technician that there's something seriously wrong with your piano that will cost big bucks to fix, you can consider a new piano then and weigh that against having your current one fixed up.
Just curious -- if you're not using the piano all that much anyway, why would you want to upgrade the piano? It's not like a Steinway will need any less maintenance than a Boston if you put them in the same environment. In fact, I suspect it'll need even
more maintenance than a Boston to stay in good playing condition. After paying the kind of money one needs to pay to acquire a Steinway L, it's natural for one to expect a lot, and that expectation would carry a price tag to match if it is to be met.
BTW, Kawai the Japanese piano manufacturer made many of their action parts with synthetic material (ABS carbon composite) to get around some of the potential wood/humidity problem. Kawai made your Boston too by some kind of OEM relationship with Steinway. But for whatever reason known only to Steinway and Kawai, all the Boston pianos have wood action in them (just like Steinway and most other pianos).

Good luck.
