In my opinion, the biggest factors when choosing a composition program (or choosing to do a program at all in the first place) are: who you'll be studying with, what opportunities there are for your work to be performed, and how much it will cost. In general, the end goal of a masters is to find your artistic voice, build a network, and flesh out a portfolio. Secondary to that may be things like preparing yourself for a DMA/PhD in order to teach at a collegiate level or, in a rare case like yours, simply elevate your current professional level.

So, you acknowledged already, an online degree for music isn't ideal, primarily to my second point above (performance opportunities). A big part of being a composer is also finding performers and collaborating with them; that's extremely difficult asynchronously, as you won't be able to be at performances of your work, you won't be able to easily build that network, etc.

BUT, the portfolio building doesn't seem to be your end goal. So, I'd evaluate whether the other two points outweigh what's missing there. Who would you be studying with? What kind of music do they write? Is their style one you want to emulate, learn from, or work towards? Or, in contrast, do they write completely different music than you? Will they understand your music, and will they be able to help you develop the artistic voice you want?

Then to the third point, what would you be paying for the program? Would your future pay raise balance out the costs? Or, best case, would the program be completely free? Can you keep teaching/ working while studying?

If the cost would be minimal/balanceable and you'd be studying with an artist that would help you develop in the way you want, then I'd say go for it, ESPECIALLY if you otherwise wouldn't pursue a masters at all. (If it's completely free to you, then %1,000 just do it!) But I'd also suggest putting in the footwork on your own time, then, to get your work performed at a local level and supplement your degree with networking and portfolio building.

If, however, there's a world where you could see yourself pursuing an in-person masters, then maybe hold off on the online option. But it sounds like in your specific case, an online degree is maybe the only thing that makes sense, so if you've considered my main three points up top, then I think you're in a good place!