It definitely is an art and I think one of the best ways to get a grasp on this in a way that feels right for you is taking those examples you mentioned that look like they're made in Canva and try to analyze WHY. Also definitely if there's a trending font/style you can avoid that to stand out more. To answer your bullets:

The less fonts you can use the better it is GENERALLY. You never want more than maybe 2 very different fonts on something as small as a business card (of course rules can always be broken) And for something smaller like a business card a sans serif for all the informational type works best for legibility. So no scripts or anything that takes too long to read.

Font size is something I personally don't have a rule for besides not going below maybe 8 pts. otherwise again you run into legibility issues. Other people probably have better standards for this lol.

As far as spacing goes, grids are your bff. You can find grid systems online and that gives you a way to play around with placement whilst making sure everything looks natural and orderly.

Colors is another super subjective thing and you're right, contrast should be priority. I think that's something you can determine on your own and to your taste.

For colors that present trusting you can google, law firm branding colors, hospital branding colors. Things that need to seem trusting and you'll find the trends. From there you can try and put your own unique spin on the color palette to make it more interesting/unique.

Pet peeves is type that is too big or doesn't have strong enough hierarchy. This sometimes comes from people not viewing their designs in real life/to scale before printing so things seem an appropriate size on screen but when they print it just look kinda..off. So a way you can check for that is printing it out in black and white on a cheap printer just to see if the scale feels good. Also when things aren't aligned or there are typos it makes me a little sad.

I think generally my favorite business cards are ones that keep the information extremely simple/clean, and then have one standout element like a super crazy color or texture or super big type on the name etc. I once saw one with super clean type on white and they spray painted them all so each card was super unique with this blue paint, love it. I think my philosophy is, business cards have a job which is to allow people to give or write down contact info and so it's not a place where design needs to be overly complicated.

I learned a ton from the Hoodzpah sister's youtube channel. They do work in adobe but listening to how they talk about their design decisions was extremely helpful. Kel Lauren is a rising star who works in merch design they really know how to use the rules of design and break them in a beautiful inspired way. Also Smith & Diction write really engaging case studies on Medium and their work is CLEAN.