I have also found booking a very long time in advance (18-24 months), the solo supplement is often 25%. Soon after I have booked, that discount disappears. I have booked Silver Ray for March 2026. The transatlantic has a 25% solo supplement, but the Caribbean cruise is 50%. Caribbean cruises always book well, so no incentive for SS to reduce the supplement. Transoceanic cruise do not book out, hence a better incentive.
My TA did pass on that email from SS with the list of low solo supplement cruises. Not that many of them are this year.