You could try only doing long rides. That should reduce internal moisture. My old Nova car did 165,000 in 8 years without replacing any exhaust bits due to a 55 minute commute to work. My Clio has done 5 years on the same system, but Covid and stand downs may see many of our car and bike mufflers rusting out!
You could try taking off the Uly muffler and pouring in a fairly liquified rust treatment, slosh it all around then drain on end, but be prepared for some toxic smoke after.
I would suggest something that will lube the exhaust valve pivot rather than seize it or gum it up. l spray half a can of WD40 up and down my XB muffler every time it comes off. It’s not a bad idea to take it off regularly to have a look around and to clean the grit out of the exhaust clamps. For this reason l reversed the two rear exhaust straps so that the clamping nuts are on the left for ease of access. I do the clamps up tight, but only finger tight with a deep 11mm socket to avoid breaking the straps, which can happen. Mine never come loose or break, but then l only have an XB9 and don’t go fast!
I spray the outside of the standard muffler every year mainly for cosmetic reasons. Have used Hammerite black, Wilkinson’s own satin black, and BBQ paint. It seems to need doing every year anyway, so why not just see it as a chance to clean and lube? I bought a can of Johnson’s VHT from Europarts for £6.59 recently, but haven’t used it yet so no report yet!