A redesigned iMac with some new components will predictably have the year-over-year improvements. I don't get the sense that you're at the limits of the 2019 iMac's performance, so I don't think there's much of a utility argument to be made.
I think the underlying question is can you live with the old design when the new hotness comes out. I get that. I'm persuaded by that too. Tough call. Not knowing how well you bought the 2019 iMac, my off the wall guess is you could lose $1,000 in a spec-for-spec, used-to-new transition if you do it at launch - there probably won't be much discounting for awhile. $1,000 impacts different people differently.
If I were in your shoes, since you just made this investment I'd press pause. Let it unfold and let it marinate. See how good the reviews come in and monitor the potential first year defects. You can always pull the trigger on it later.