Good overview! I'd personally rather have better tooling for upgrades. Recently the API changes have been minimal, but the real problem is the mandatory node draining that causes downtime/disruption.
In theory, there's nothing stopping you from just updating the kubelet binary on every node. It will generally inherit the existing pods. Nomad even supports this[1]. But apparently there are no guarantees about this working between versions. And in fact some past upgrades have broken the way kubelet stores its own state, preventing this trick.
All I ask is for this informal trick to be formalized in the e2e tests. I'd write a KEP but I'm too busy draining nodes!