Or instead of router mode, try bridge mode feeding into any old pc running
Noting that even some crappy hardware will still fall over when put in its so called "bridge" mode, which should just be some packet buffering between the wires and their encodings, but it's obviously still looking at the traffic above layer2. So you may still need to swap out hardware.
because there is secondary storage (HDD and/or SSD), pagingis available if the routing functions' memory needs grow larger than the
Sure, but there's no free substitute for RAM, and you probably don't want packets burning a hole in your SSD. Add more RAM if not maxed out. disable swap, boot USB, set read-only, use small ramdisks for write paths. If used RAM for a used PC isn't in budget or isn't enough, then maybe spindle, but it won't be as fast. And eventually CPU or interrupts or i/o get swamped. Then you put a newer PC that can hold proper amounts of RAM, CPU, etc.