My jeans are part elastane too, but I honestly feel more comfortable in properly sized straight cut 100% cotton twill or wool(-blend) pants with no stretch, preferably with pleats. Elastane lets you get away with a fit that's really too tight.

But none those are even close to being as comfortable as pajama pants or sweatpants. When I'm lounging at home, I want nothing to bind or sit tightly, comfort is king.

Wearing a belt while sitting down puts a lot of pressure on your midsection, but even wearing suspenders (which I highly recommend), you still feel the waistband. Of course you get used to it, but why not just go straight for comfort?

I second your suspender recommendation (They're called braces in the UK). I find them much more comfortable than belts, and they work better than belts if you have developed a bit of middle-age spread.