On a slightly different topic, we’ve got two episodes left to find out if Zemo goes after Isaiah Bradley. He was definitely listening while Sam and Bucky talked about him on the jet.
Also, if Old Cap is still alive and Zemo finds out, he’ll probably want to finish him off, too. Zemo on the loose is bad news.
Zemo is a bit of a fanatic, but he seems more than capable of prioritizing. He works with Bucky, whose existence he despises, to go after what looks like a bigger threat - the Flag Smashers and the serum.
In terms of threats, I don't think Old Cap and Isaiah are high on the list. They are old men, inactive. The threat they represent is only as a source for data to make more serum. I expect the world will always have enough active super-powered threats for him to be concerned with that the inactive ones will simply die before he gets to them.