$20M/yr, 3 devs, "And post COVID, budget is really tight"
WHERE IS THE MONEY, LEBOWSKI?!!?!
Seriously, WHERE IS THE MONEY GOING? I'm all about keeping a tight small team, but where is the money going? Even paying for a manager to help them move in a direction and address business risk would be worth the investment.
If you really have that level of revenue, and only 3 devs, then you need to be looking at the risk of losing one of them.
All the tech debt is irrelevant, your focus should be on mitigating risk due to attrition/burnout/mistakes.
That being said, just getting a sane deployment process would be helpful.