It'd be interesting to see a version of this that tries all the different interleavings of PostgreSQL operations between the two (or N) tasks. https://crates.io/crates/loom does something like this for Rust code that uses synchronization primitives.
There would be some challenges for sure. Likely optimistic concurrent patterns would require an equivalent of loom's `yield_now` [1] to avoid getting stuck. And you'd probably need a way to detect one transaction waiting for another's lock to get out of situations like your update lock vs barrier example. I vaguely recall PostgreSQL might have some system catalog table for that or something.
I've built something similar to Loom, except it's more focused on extensively modeling the C++11/Rust memory model (https://github.com/reitzensteinm/temper). My experience is that fairly shallow random concurrent fuzzing yields the vast majority of all concurrency bugs.
Antithesis (https://antithesis.com/) are probably the leaders of the pack in going deeper.
It uses generators and their yield as the yield point (and supports running arbitrary functions under a debugger)