[EDIT to clarify: here I’m talking about writing a javascript based extension, not a content blocker]

That’s not my experience (in the admittedly only browser extension I’ve ever written).

After getting it working to my satisfaction in Chrome and Firefox, I created the Safari macOS/iOS versions by running

    $ xcrun safari-web-extension-converter --bundle-identifier <bundle ID> .

in my repo. Then I opened xcode, configured the signing/capabilities, and built it. IIRC I had to create the directory for the output because xcode didn't do it itself, but once that was done I could install it to both macOS and iOS. Honestly I was surprised it was so little effort. I don’t doubt that an extension with more functionality than mine might require a jumping through a lot more hoops, but it definitely can be easy to successfully target Safari IME.

Interesting. Maybe I tried to do something too complex, I don't remember.

https://developer.apple.com/documentation/safariservices/cre...

Here they talk about the "container" that needs to be in Swift/Obj-C. I remember I used to have some problems with the "container" application

I think the issue starts to be when you want your extension/app to have any kind of settings that is retained? I don't remember, really.

Yeah, I didn’t go the “Creating a Safari web extension” route, rather I went via “Packaging a web extension for Safari”: https://developer.apple.com/documentation/safariservices/pac...

I note that they say the command has changed from the one I used, it’s now:

    xcrun safari-web-extension-packager

My extension has storage/settings, and it works fine. I have edited no Swift code at all.