From what I’ve seen and read, server to server traffic is less taxing on instances than client to server. So even if your instance is JUST you, it would be your instance talking to everything else so it would have some net benefit on the federation. But it would take a lot of users self-hosting solo instances for this to help in any noticeable way, I’d think.
There is certainly no downside to running a solo instance, if you’re even slightly interested I would say go for it!