I'm in a similar position - submitted to 11 agents, 4 rejections so far (of which one was very complimentary). I'm still waiting to hear back from the other 7.
I'm not stressing about it. They're busy people and, as you say, their slush piles have been swollen by lockdowns. If I haven't heard anything back after 16 weeks, I'll assume they're not interested. I have my next 10 agents already lined up, then the next 10, then the next 10....The author Will Dean advises that we should expect 100 rejections; that way we will breeze past rejection number 23 without it knocking our confidence.
In the meantime, I'm polishing the book so that if anyone requests a full MS I feel it's in the best possible state. When I'm not editing, I'm working on the first draft of my second book. I'm in no rush..what will be will be. If the book's good enough, I'll find representation eventually.