Personally, I think you should either keep the 2080ti, or sell it someplace like Ebay. I finally just got my RTX 3080 from EVGA, which now replaces my 2080ti that I had previously. So I'm torn about what to do with mine also.
On one hand, I'd kind of like to keep it for awhile in case the 3080 ever has issues. And I don't believe a person can even buy a new 2080ti anywhere, only used. And the used prices I've seen range from $900 to $1200.
So on the other hand, selling the 2080ti, I could make more than what my 3080 cost. You could do the same. And trading it for a 3070 straight up (even though that's a newer card), you'd really be loosing money. Buy a new 3080 (and I know that could take months), then sell the 2080ti and you'll have a better card, with a warranty, and several hundred dollars to boot.