How about metal core PCBs? [Example](https://www.multi-circuit-boards.eu/en/products/printed-circuit-boards/metal-core-pcb.html). I've used them for high power LEDs, and we etched in house using basically standard processes.  Of course the do place limits on your design (and they're annoying to hand-solder), but if you can get away with double-sided boards they'll give you a solution that will disssolve in anything your Al case will dissolve in.

The insulating layer is typically 100 µm thick.