Seems like Flower and Kaleth have steered you in quite the right direction, but I'll add my two cents on some things.

pat_furring said:
I use Perchance to generate my images

Just about nothing saved from Perchance is of "e6ai quality".
It does the bare basics of AI generation, and lacks the tools that most directors use to refine their images - namely inpainting. It's also a problem that I understand exists on CivitAI's webgen, but that's neither here nor there.
However, we have absolutely zero problems if you take your gens and edit them externally - in fact, we encourage people to do that.

I want to create something amazing that as many people as possible can enjoy, but at the same time, it’d be a waste to fill up space with content that the majority of users aren’t into or just ignore

This is why we have a blacklist system (as Kaleth mentioned)
Sure, it oftens seems like there are some who choose to ignore the fact that it's there, but it is, and people do use it.
Also, just as an aside, you mentioned vore and blood, so you've vastly limited the views there. For those who do utilise their blacklists proper, I can almost guarantee that those are two things quite commonly found on the average user's blacklist.

... if it’s technically possible for someone to change a thumbnail

No. I'm not quite certain how the video thumbnails are determined, but I'm positive it's not the first frame (judging by some of my own video uploads)

I mean users who don't click on those, being like ‘Ugh, those spammers are cluttering up the board again'

We have rules against post spam in terms of prompt spam or low-effort edits, so such posts get deleted anyway. If you've familiarised yourself with the guidelines, then no doubt you'll be golden on that anyway, so don't worry too much!

Image Resolution

If you want the exact guideline, it's in the bad things to upload section and it is specifically

• Small images: Anything below 768px in either dimension is likely to get deleted.

This means if the width OR the height is below 768px, deletion is likely. There are a few cases where exceptions are made, but they're not common.
Also note that this only applies specifically to images and not videos (at the time of writing)

Upscaling

This is in the terms of AI upscaling. Of course we allow that, but we try to make it clear that doing this often makes what might've previously been practically invisible mistakes/anomalies a LOT more obvious to us, or even create new anomalies in an image. If the effort is taken to remove those, the less likely a post is going to get deleted for "bad use of upscaling"

"stretching."

I should note that just by using any image manipulation software and using the "resize" function is probably going to deteriorate the quality of an image in a way it's just gonna get deleted for being low quality. The way Kaleth described of doing that, then putting it through img2img on low denoise is usually the safest bet (followed by inpainting whatever anomalies that may occur from doing that)

It’s videos/AI animations. They can definitely be mistaken, but with the lighting and image style, it looks more like a movie or commercial shot than something filmed in real life. But I’ve got my answer now - it’s not allowed unless it’s a dragon. And if the woman looks real with a clearly visible face (even though the face is AI-generated, probably not based on any single real person), it’s still a no. Got it.

Okay this has been misinterpreted or miscommunicated.
The specific rules are no photorealistic animals in suggestive/explicit situations, and no photorealistic depiction of human faces.

blood

Refer to my previous blacklist comment.
 
 
 
Now, I mean this in the most nicest way possible, but you have to have some thick skin to post on here.

Some of your posts WILL get deleted. It happens to practically everyone at the start, and often times it sends people into some of the wildest crashouts. You don't have to go too far into the forums to find some of those posts.
But us staff, and a good handful of the community will help you out with ways to bring your posts up to "e6ai quality".
You said it yourself:

It's in everyone’s best interest to support the most talented AI creators.

and there are people here who share that similar mindset. Heck, just by even saying that (and the fact that you asked for SO MUCH clarity) means - at least to me - that you'll do just fine here :D

People WILL downvote your posts (especially those who don't use their blacklists). My best advice is not to pay attention to the numbers - EVER. It'll drive you batshit without a doubt.
I'm with Kaleth here

kalethorebiter said:
Trust me, on the internet, you will ALWAYS find someone that somehow is upset about what you do. Don't listen to them, do it for yourself and the pleasure of sharing, not for the haters.
The more important is this: Take care of yourself, do stuff that brings light in your life, and let the sour crowd macerate in their own bile.

Haters are always gonna hate.
As long as you do it because you love doing it, then keep on keeping on, my dude.

~ Sparkz