There is an parsing/ast_helper module with Pat module with the following not-smart-enough function:
val construct: ?loc:loc -> ?attrs:attrs -> lid -> pattern option -> pattern
You should indeed feel free to send a PR to add
val construct_n: ?loc:loc -> ?attrs:attrs -> lid -> pattern list -> pattern
I see a couple other improvements (Texp_construct would benefit from the same helper, Texp_extension_constructor needs help, exception patterns can be improved, etc.) from looking at Untypeast this way, but nothing that would clearly require a fundamentally different approach.