I will try my best to give you the most accurate answers and I hope people will correct me if I'm wrong.

First of all, for your current website sales, I don't think there's anything you can do differently unless they offer APIs where you can control/customize your account by linking it to nostr.

One way that can fix ALL your difficulties at once is to use BTCPay Server on your own website. BTCPay Server will be your all-in-one point of sale as it can work online and off. It can also handle all lightning and on-chain transactions. Using this free service with an additional pay wall on your website will allow you to achieve almost everything you were asking about.

As for nostr, more and more apps and code are popping up weekly that help link your nostr notes/account to your website.

Otherwise, you will need to see what gallery software you can use in nostr or just try to provide very low resolution versions of your art here in notes, where people can zap you for the high-res versions.

What we are using nostr for currently is for social interaction but we are slowly adding new ways to interact with each other's in other ways.

In my opinion, your best solution is through the free BTCPay Server but setting it up on a tor node is pretty difficult. However, there are a dozen or so providers who offer it to you without any need to install it yourself but you will have to pay for that.

As for limited edition art, I'm not that familiar with how that should be tackled.