I would not think that it would be very stable. I have not seen it for sale recently, but that may have changed.

The Metol equivalent of Rodinal that you were working on is very similar, if not identical to Kodak HDD, a developer sold many years ago and described in Anchell and Troop. It was quite stable.

However, plain Metol in Sulfite and Hydroxide is not. Among other things, it can form Sulfate precipitates which is why I assume you are trying to get the free base of Metol. This would solve the occasional precipitate problem and the relative concentration problems, but not the stability problem. This needs additional ingredients as you have probably already found.