If you have a ons or short relationship you have no idea who that person is. Will they make a good coparent? Will they work with you or against you? Will they want 50:50 to avoid CMS, despite palming them off on latest new partner they have moved in? Will they accuse you of neglect or refuse to return the child? Will they use the child as a weapon against you? Will they make the child miserable or happy? Will they be safe or not?
If you are going via sorry donor, you know the plan. It’s all on you. There’s no chance of the donor turning up wanting to see the baby on and off through its childhood, disrupting the child’s emotional well being. Plus there’s screening.
I would say a short relationship or ONS might be preferable IF both parents were guaranteed to coparent well and respectfully of each other and always put the child first.
But it rarely happens like that because the parents usually end up resenting and disliking each other.
I have also been a single parent. My exh had a mental breakdown. He wasn’t safe to live with. He got help and started seeing the kids. Coparenting with him was extremely hard. He tried turning the eldest against me by telling her I left for someone else (I didn’t). The youngest would ‘lose’ things like school coats or trousers or trainers all the time and exh wouldn’t replace them, so it cost me a fortune. He would keep them off school or drop them late everyday, which meant both of us ended up being spoken to by the school. He was self employed so no CMS. When the kids decided to not bother with him as they got older, it was actually easier to have them here all the time.
Being a lone parent, is far easier than coparenting with a difficult ex who can not put the kids first.