@BeanieofJustice You're doing such an amazing job!!! He looks so good and I am loving the pictures of him!!
One of my favorite things about baby kittens is watching them start to groom themselves before they can even walk, I'm looking forward to you seeing that!
I started to put a list together of supplies I usually find useful but then it morphed into a list of additional care tips in general. So I'll post it here just in case there is any helpful new info for you but you're doing such a great job already, and I can tell you've done a ton of research and have probably come across a lot of this already. I don't want to overwhelm you any further so take this all with a grain of salt.
1) Puppy pads are also WAY cheaper on Amazon than in pet supply stores.
2) Tapping gently at the anus/genitals to stimulate elimination works as well as rubbing and can help avoid the inflammation we see sometimes. But then again, mama cats rub and they do it with a barbed tongue so it probably doesn't matter that much. LOL
3) Their immediate surroundings should be about 85 F when this young. So you're doing the perfect thing with the supplemental heat with room to crawl away from it when needed. I would continue this for 2-3 weeks at least. To avoid the ability to crawl away too far from it, I usually keep them in a cardboard box lined with puppy pads and towels.
4) If you have a gram scale, like for food, that's awesome. Your kitten is doing so well I'm not sure you really need to go buy one unless there are signs of trouble. But with a litter of neonates I usually keep a chart of daily weight. General guidelines are 100g at birth, then 10-15g (on average, there will be fits and starts) per day. After about 3-4 weeks I stop weighing except for deworming purposes, since their survival rate gets much better if they make it that long. Weight can be useful for keeping tabs on whether he's eating enough since it is often the first sign of illness. (The PDF I mention below has a chart for how much they should eat at each stage.)
5) As someone else mentioned, eyes begin to open at 7-10 days. And deworming with pyrantel should start at 2 weeks of age, then every 2 weeks after that. You can take him to your vet for that, but if you are able to weigh him at home, some vets may give you the amount you'll need for the first couple of months in a small bottle with an oral syringe to measure it out. Just be sure to get it from your vet though. You can buy it OTC I think but I'd worry about the formulation and quality. As for whether he needs deworming, he totally does. Most orphans come from homeless mamas who eat random things, and infections are usually passed in utero.
6) You asked about a litterbox. When he is reliably walking around on his own and you start to see pee spots on the puppy pads, I'd start with maybe a small pan with low-ish sides, like maybe a small baking pan, and put a shallow layer of litter over it. Super important is using a litter that wont clump hard, I use Worlds Best, which is corn based. If he eats it, it's not great but at least it won't form a ball of concrete in his stomach like the clay-type clumping litters will. If he eats it, take it away and maybe try a different one, like a paper-based one. When he is big enough to climb into a litterbox, I'd start using one of those super-cheap small low-sided open boxes. That will get him through his kittenhood just fine.
That's it for now. I hope that wasn't too much of a repeat of everything you already know! I also have a very informative PDF presentation put together by a veterinarian on orphan kitten care, but it's too large to post here. If you want to PM me your email address I'd be happy to try sending it that way.