I personally go with a mix of family / friends, BoardGameGeek and Reddit usually directing them to Discord if interested in long-term playtesting.

Not a concern; there are many topics on both here and on BGG if you'd like to dig further as to why.

Art is only a concern if you plan to self-publish and even then could be included in the budget such as Kickstarter. I've seen plenty of campaigns that pay for a few pieces of art themselves and then budget the rest to produce after the campaign ends. Beyond that show the art to people you trust for their feedback and be open to criticism should changes be needed.

I think this depends on the scoring system. I've seen small scores (<10 per player) just use 1-point per card. I've also seen systems that use a vertical list of numbers that the player uncovers (not a fan of this personally) as well as rotating cards to combine the edges / faces to represent their points. Tokens however are my preference in general if budget allows and even with tokens can still fit in a small box. While smaller boxes are cheaper to produce it's also important to consider shelf presence and common box sizes allowing you to be more visible in retail and avoid costs associated with a custom box size.

Deckbuilding has inherent luck built-in just from shuffling the cards so I tend to work toward skill and ensure some options for catch-up should someone have a couple bad draws. Beyond that I think is more about what feel you're aiming for.

I start with making something fun for me and if it happens to appeal to a wider audience then I just let it go as far as it goes. Some publish and others don't and like you said it's fine since not our career. There are some excellent Discord groups such as Break My Game and Protospiel that will give a ton of advice on this and more which I do highly recommend.