I'd call the web interface on low-end managed switches a liability [0]. It would be interesting to write one's own 8051 firmware from the ground up [1]. It shouldn't actually be terribly hard to have some basic thing that accepts a binary chip config image from the network, right? The existing 512KiB flash ought to be enough for that. And since Realtek switch chips seem to be so popular, it could even be made generic enough to work across models. Then a user would just need a flash programmer.

My core network is Mikrotik gear with 10Gb uplinks, but it would be nice to use my old unmanaged gbit switches (Netgear GS108 mainly) with vlans rather than going nuts with more Mikrotik or having lots of homeruns.

[0] high-end ones too, for that matter

[1] the alternative I thought of first was setting up an Arduino as an I2C slave. But then you'd also want to switch the switch's power supply, and need an ethernet port on the Arduino just to connect to the switch itself.