First custom. Need paint advice.

for straps and belts, you can also you rex lace. available in different thickness and at most hobby stores. it also takes paint very well.
CAboose

For straps I use one of two things, depending on how they fit the body. For straps that wrap around the circumference of a body part, I use plumbers shrink tube you can get from any hardware store. A hair dryer or heat gun will make it grip the body. For straps and other parts that are just placed on the body, I use clear vinyl you can get from a fabric store. That stuff has so many applications, it's ridiculous. Just cut it to size.

I concur about rubber bands, while they're okay in the short term, over time, they will eventually degrade, snap, and fall apart (it's possible that there might be some kind of sealant that would prevent this, but I'm not aware of any). I use paper, layered for thickness, to make straps, but then, I use paper for everything. If you can find some nice pleather, or similar material, that's in scale, you can cut that down to strips and glue those onto your figure.

Avoid rubber bands if you can. They tend to degrade with age and become brittle. They are also affected by paint and sealant.

For the paint soak the figure in mineral spirits for a few hours or over night (will not harm the figure). Then use a paper towel or something to rub the paint off (tooth brush will also work). For straps, mr sinisters cape works very well. Also for paints try model masters instead of testors. They are slightly more expensive but worth every penny. Hope this is helpful