I get the desire, but
it seems unprecedented. Only Open vSwitch seems to be “just a switch,” and
a fairly advanced one at that. All the vendor options seem on par with RouterOS, more or less.
Find me, let us say, a Procurve/Aruba 2540 image, for example. Find me a TP-Link JetStream image. Etc.
It looks like if you want an L2 managed switch, not a router-with-switching, it’s vSwitch or nothing, at the moment.