Getting non played sims into relationships/marriage, can you?

You are absolutely in the right place to ask your question. You've also asked about my favourite subject!

I'm on Xbox Series S. I find that the easiest way to do this, rather than pair a couple of NPCs/townies together is to just add a partner yourself to an NPC household. So take Bob Pancakes for example, he has a wife already but if he was solo I wouldn't take say Holly Alto and add her to his household then edit their relationship. Instead I'd create a spouse for him myself and then edit their relationship to make them married.

I also keep some townies as roommates and I regularly add my own Sims to a household like this, the BFF household for example. I start my Sim out there as a friend so that they begin the game with friend relationships. I also created a sister for Summer Holiday. Sometimes I leave my Sims there for a while and then go back later to split them out to play. While the house is still in Other Households and unplayed, you can edit and renovate the house to your taste and requirements at no cost to your Sim, then add your Sim into the redesigned swanky pad with instant buddies. You'll still need to build their friendship up as it starts out as acquaintance unless you've made them a relative. Sometimes I move some roomies out and sometimes I later move my Sim out. It's just a starting point if my Sim is a solo but social Sim and it stops those sadness moodlets from a new lonely Sim who has yet to make friends. It means they immediately always have someone to call to chat.

Another thing I sometimes do is create a young adult child of an NPC household. So for Aliki and Mele in Sulani for example, I added 2 daughters. Gigi is their daughter and Pia is just Aliki's. I added a Husband and Son for Gigi then moved them to their own lot as her husband Keanu is an island guardian. I added a sister for Keanu and moved her to her own lot as a fellow island guardian. I moved Pia to her own lot and then later added her Husband and daughter. I made a complicated family feud situation as nobody gets on with Aliki...and he never stops calling but nobody picks up!

I've only had problems with this occasionally and randomly where it wouldn't allow me to edit pre made Sims, usually it works fine for me and with these households I've never had a problem.

Another thing I do is to keep a group of townies and NPC Sims in regular circulation as friends/love interests for my Sims. I edit everything about them to make them love interest/friend material. I often take their houses for my own Sims but I leave them as Currently not in World and in Other Households. I regularly cull random townies as I find the game adds too many and it can slow the game down, Cottage Living caused a real problem with this. I keep Other Households between 70 to 100 Sims.