You asked
Should I upgrade from the sony a6600?
I can't say for sure whether you should upgrade, because this site doesn't compare the a6600 with newer bodies. It's still a capable APS-C mirrorless camera with in-body stabilisation and a big battery, so there's no rush unless something specific is holding you back.
Often the better money goes into lenses rather than a new body. The Sony a6600 page lists every lens that fits it, from the Sony E 16-55mm f/2.8 G for everyday zooming to the Sony FE 70-200mm f/2.8 GM OSS for sports and wildlife.
If you do want a new body, buying used is the cheapest way to get one. Tell me what you mainly shoot and I'll be more specific.
Written by Claude from this site's pages.