It's a common narrative that it's harder to be more successful as a solo founder. I don't agree because actually 66% of startups fail because of cofounder conflicts. Also startups with single founders tended to last longer and eventually achieve higher revenue (52% startups with succesful exits is solo founded). So if you are a solo founder, your risk of failing goes down significantly.
As for having someone to share your struggles, I joined this solo-ish founders community called https://FoundersCafe.io and they are bascially my co-founders without any of the drama.
I also hired out all the skillsets I lack.
Being a solo founder is fucking awesome as long as you have your needs met (emotional support + skill gaps)
Most often than not, they drag you down when incentives don't align.
I'm not too sure about _absolutely_ selling before making, but definitely do a market research and check if you can find people who would, at least theoretically, buy the product you intend to make.
If you make before you sell, you're saying that you are a visionary and people cannot understand your genius. Those are not odds to my liking, but if you like them you do you.