I’ve had instances with only a dozen or so Wordpress sites that will chew through 8gb and go into swap when certain tasks were poorly scheduled (like a couple of sites clearing their cache at the same time).
So I’m not surprised they are hitting 25gb but the gradual nature (from the sounds of it) off the memory build up does sound like a leak.
Don’t envy the devs but I’m sure some of them are enjoying it 😂
When you say it’s going to ‘take forever’ how long are we talking? Can you try a small batch and see how long it takes? If you really are only doing this every 5 years or so and don’t want to spend any money then the setup you have may be your best option.
If you want to spend $10 or so you could buy a cat6 Ethernet cable (and you may also need to buy a dongle for the laptop). The transfer would take a few hours.
Depending on your internet speed you could also sign up for a single month of Dropbox and do it that way (again, about $10).
For the quickest (and most expensive way) you could purchase an external nvme/SSD which would do the job in a few minutes. Couple hundred dollars but then you’ve got a very useful device that you can use in future.