A quick thought on villainous motivation. Everyone who writes these days seems to get that your baddie, or main opponent, or whatever you want to call them so that you don’t feel they should be twiddling their moustache, needs to have a reasonable motivation to do what they do (for a given value of reasonable. I consider it a perfectly legitimate comedy motivation that the character is trying to be as villainous as humanly possible, because otherwise, Lord Nasty will make some very snooty comments down at the club)
What gets less of a mention is that it’s often interesting if your villain’s motivation ties in to your hero’s. At a minimum, what they want should be the thing that creates conflict with the hero, but why they want it can create whole new layers of meaning.
One basic tactic is to give the villain a very similar situation, but have them pick a different approach to it, making them as much mirror to the hero as opponent. They should offer a different angle on the central theme of your novel. Possibly a diametrically opposed one, but it can also be fun if the difference isn’t that great, or indeed is only one of degree.
One way to check for that is to ask yourself whether your villain’s reason for their actions sounds plausible enough that, for a moment, you could consider your hero agreeing. It’s not necessarily an outright plea to join the dark side, but there should be that moment of understanding somewhere.
Not least because it is that understanding that makes your opponent so well placed to oppose your hero. Remember that the main antagonist in your story is the person best placed to expose and attack the fundamental weaknesses of your main character, not necessarily with an army of crab monsters (though they can be fun) but by encouraging them to give into those weaknesses and see the world their way. For that, you need a way of seeing the world that has at least some connection to the hero’s