Honestly, what's left looks harmless. I wouldn't worry about it.

That looks like "seasoning" you'd get on baking sheets and cast iron. Seasoning is polymerized oils that end up having a high temperature breakdown point, quite a bit higher than the original oil. It's not an issue, and if you're willing to put up with it, might actually make it easier to clean since it has non-stick properties.

Otherwise, you want to use a basic (as in acid-base) cleaning agent, ammonia, oven cleaner, etc. should work; bleach might not be a strong enough base. Oven cleaner is made for cleaning this type of thing, but it's one of those cleaning agents where the precautions are absolutely required, not just a company liability thing because idiots.

In my opinion, if soap and water and scrubby pad don't remove it, it's not worth further effort.

Rag damp but not dripping with ammonia sitting on greasy spot over night should loosen the grime and allow it to be washed off. You'll need to put it in a trash bag to keep the smell in.

I've used ammonia in a trash bag for greasy oven racks.

That looks like the fan for the convection part of the oven.

Good luck. You can clean it, but it'll look just like that again in no time. That's not really dirty for an oven, in my mind.

Know why commercial ovens look like they do? There's only so much cleaning you can do in it's down time.

I wouldn't leave puddles of stuff in an oven, just that they're going to get coated/caked/baked on, especially since these consumer ovens use a cheap sheet metal inside, which is very hard to clean or keep clean. It's not stainless or an enamel, which are very hard, non-porous surfaces, which makes them easier to clean.

Most likely the owners manual says to clean it just run it on max for a period of time. That will carbonize whatever is caked on, making it easier to scrape off.

I would try hot water with vinegar. Depending on the need, 1:10 parts vinegar-to-water should be enough. The water can be as hot as you can stand.

(I’ve used as high as 1:2, but that’s break-out-the-gloved territory, and be mindful of the fumes.)

(Works great to strip seasoning on cast iron, too. Here just bring it to a boil in a pot and then let it cool off for soaking.)

I know you're apprehensive about using vinegar, but it is quite a potent degreaser. And I mean, don't give it a bath, but wiping it down with a rag or a toothbrush shouldn't be too bad. That's how I clean my air fryer

You could try something like isopropyl alcohol if you're concerned about the electronics. If you use a high purity then it actually evaporates on its own and is non-corrosive. It's my goto cleaner for electronics parts.