I could, you know, go to the office, but everone interrupts me there. It's not lonely, but unproductive.
The trick is to be productive, but to feel like a human in the mix. For me, Starbucks does that.
Some kind of optimized noise is a weird concept, but I've found that it really works, from time to time. Especially if I'm doing something that requires a great deal of concentration rather than just "production".
The core of the UX is the Focusmate session, a structured, 50-minute video interaction, where you and another user act as accountability partners for one another.
You'll find lots of solopreneurs as well as a handful of sole founders, but above all, you get human interaction without compromising productivity.
Disclaimer - I'm the founder. I was a sole founder for a lonnnng time and doing these sessions was my saving grace.