I use sanding sticks (the kind warhammer and model builders use) and my micromotor tool to sand extremely small bits. It's worth it to get some fine grain sanding sponges as well if you don't already have them. They work great on smooth round surfaces.
The micromotor tool I use is called "Belle nail drill", and you can buy all kinds of great attachments for it.
Belle Nail Drill Electric Nail... https://www.amazon.ca/dp/B07ZCHTDMS?ref=ppx_pop_mob_ap_share
super sculpey is a tenner for a pound of clay (lb) on amazon, you can pad it out with silver foil/wire.
There’s also a wire called high fire wire that can go through kiln temperatures, I’m sure that would work well in an oven too. This is the one I use Kemper High Temp Wire 17 Gauge 10 Feet Great General Purpose Support Wire (Original Version) https://www.amazon.com/dp/B00CLC5JWO/ref=cm_sw_r_cp_api_glt_i_3RC0BMZP7CRMM56AC3E1
I use acrylic paint over polyurethane primer. Specifically, i use Vallejo primer through an airbrush. I use high-quality model acrylics but any acrylic paint will work ok; Vallejo model colour is an excellent choice or vallejo model air for airbrush (though any paint can be used through airbrush if you thin it down correctly). Check out elementgames.co.uk as a good supplier for model-specific paints and tools, I think any of the paint ranges they stock would be good for resin sculptures.
Regarding airbrush, I started on a cheap one and found it fighting me at every stage. A good airbrush is so much nicer to use. I have a Harder and Steenbeck evolution and a cheap Aldi Workzone compressor.
Let me know if you have other questions and I will try to be helpful.
I haven't done a ton of sculpting so maybe I'm not the best person to advise, but the way I like to start is exactly how you've done it. Next I block in the skull, rib cage and the pelvis - just the basic shapes to dial in the proportions, I don't detail these parts. This is also a good time to make any last tweaks to your pose. Let that harden, then lay in the main body shapes - the flesh over the chest and hips, the belly/waist, neck. I build out the arms and legs by creating "muscles" out of putty and putting them in the right place, then smoothing and blending them so they look like an arm rather than a bunch of tubes.
I really like Apoxie Sculpt. I find it a bit too tacky to work with right away, so I will start a kind of assembly line of bits of Apoxie that I blend together in 5 or 10 min increments, so I always have some that's at the right consistency. I also have some silicon-tipped sculpting tools that work really well for detailing.