Does Guix fit your criteria, perhaps? If you haven’t heard of it, you can think of it as Nix with a Lisp frontend.
I unfortunately am not very experienced with containerizing packages so I can’t say much, but I know you can do it; the Nonguix channel employs containers for some proprietary software.
Like Nix, Guix has all that building-from-source stuff you’d want from Gentoo. There’s recently been work on making parameterized packages (the Guix equivalent of USE flags) a thing, but it’s still work-in-progress.
Ignoring the steep upfront cost of learning it, I’d say Guix makes it incredibly easy to add your own packages. Here’s the custom packages I currently have in my dotfiles repository. I can import one to my main config file, add the package, and it gets included in my environment the next time I reconfigure it.
As for patches, I can’t make any comparisons since I’m not familiar with Gentoo, so I think a code snippet is probably better for you to judge if you’d like it.
Here's a minimal example:scheme (define-public custom-pkg (package (inherit pkg) (name “custom-pkg”) (version (package-version pkg)) (source (origin (inherit (package-source pkg)) (patches (list (string-append (dirname (current-filename)) “/fix-some-thing.patch”))))))) EDIT: Here’s the less verbose version, which you can use instead if all you’re doing is adding patches. scheme (define-public custom-pkg (package-with-patches pkg (list (string-append (dirname (current-filename)) “/fix-some-thing.patch”))))
Not sure if this addresses your concern about multi-architecture support, but the Foreign Architectures section of the manual discusses what you can build to.
EDIT: So I was curious after posting this because usually the CLI often has much less verbose options (like –with-input for replacing inputs), and I started wondering if there was any procedure that would make this simpler. Turns out there is :) I’ve included it under the example. Although, I suppose I should have mentioned you could write your own if you really wanted to.