Traveling solo last year on Silversea (not Silverseas), I couldn't disagree more about welcoming solos. There is a cocktail get-together nightly, and some, not all, go on to dinner together. Viking treated me as a solo as persona non grata. Regent I haven't sailed since Covid (have a cruise planned this summer), so can't comment.
It sometimes takes some preemptive activity to arrange solo dining with these lines. Seabourn, I agree, is the best. Very pro-active, printed invitations almost daily if you request it. Accept or decline, easy-peasy. I'll be on Venture, first time, soon and am anxious to see if the invitations still come 🙂.