Also, a lot of pressure on Johnson comes from the press and opposition claiming that any competent leader should be able to achieve things that are likely impossible and that no-one else in the Western world has, like solve the cost of living crisis, prevent large numbers of people from dying of Covid, and stop new Covid variants from coming into existence by vaccinating the world. It's amazing how many people falsely think the UK has the worst Covid deaths per capita in Europe rather than middle of the pack, just because all the headlines carefully cherry-pick whatever narrow figure makes the UK look worse. Then there's all the endless articles about how much other European countries are doing to protect people from high energy costs which mysteriously neglect to compare how much they're actually paying for power. And of course, the current vaccines can't stop new variants both because they don't stop Covid spreading nearly enough and because most of the unvaccinated global population already has natural immunity anyway, plus the UK doesn't have a pharmaceutical industry big enough to supply the world (we're not an empire anymore!) Any successor to Johnson will have expectations placed on them that they couldn't possibly live up to, which is probably why no-one is keen to try and take his place.
There's also been a lot of attacks on the government lately for "wasting" money by buying more PPE than ended up being used in the end. Imagine trying to do any kind of emergency planning when the political climate is like that.