That's a good link, but midterms are 18 months away. What do you mean they 'happen first'? Do you mean they have to take place so impeachment proceedings become possible? I disagree with that contention.
The constitution is so vague on what constitutes and impeachable offense that it's pretty much whatever congress decides at the time.
There is enough of an argument that he's violating the emoluments clause that if congress wanted to they could almost definitely use that as grounds for impeachment.
https://swingleft.org/