Do you know of tubes that are fully flat-proof? I am looking into whether there are inner tubes that are fully flat proof.  Do you know how much they would affect performance and comfort?
Yes, they're usually solid rubber and they're very heavy, hard to fit and have a lot more rolling resistance than you're used to. Companies like "airless tyres" make them, and I suggest trying to fit them yourself before taking them to a bike shop and paying whatever they ask to have them fit the solid tubes.

You can also get puncture resistant tubes that have thicker walls than normal tubes, and while they are heavier than normal tubes they're nothing like solid tubes. They also don't work very well. You're better off with puncture resistant tyres.

Finally, you can get sealants that you pump into the tube that will leak a little then seal, like Slime. Tubeless tyres use a similar sealant, but swapping to tubeless is a major change and there are many sites describing that process