JUCE if you plan to sell commercially and make more then 20k
https://juce.com/get-juce/

HISE if you are not releasing open source. Commercial or not. If you are publishing source under GPL, you do not need a license.
https://hise.dev/#license

• when you sell you need to cover sales tax. This is hard to set up on your own so the fastest way is to find a MoR (merchant of record) to handle the sales tax. Gumroad is pretty cool and is a MoR nowadays. It's also a good place to look for and find more then experimental plugins. So signing up there is a no regret for you. Unless you want to skip taxes and just do a couple of sales, in that case setting up a small website yourself is the way to go and just do some kind of PayPal transaction
• Licenses. Keep in mind that Juce requires a license when business exceeds 20k and Hise requires a license if you don't release open source
• copy protection / license activation. 2 good options are Hise activate or moonbase. The latter is also a MoR but it's still difficult to set up under the current Juice version. It will take some work integrating either.
• installers: I think this is a bit of a hassle with code signing. David is preparing a course on this I think.

Mind you this is just my research from the last couple of weeks. I didn't go to market yet. So I might be missing some stuff or getting it wrong.

Btw for your more experimental plugins, probably the best way is indeed gumroad, and maybe just skip copy protection. Consider publishing open source as well. Unless you want to start a big ad campaign, publish on Plugin Alliance and take a more commercial approach.

Good luck!

It's not perfect but for what they offer and the hassle they save me, I find the ~12-15% fee easy to live with.

Whatever platform you use, you'll find it easier to sell if you have an existing audience.

Cool looking plugins! Do they use web views by any chance?