PS. it also seems strange that the swap usage is exactly the same - as if it's not even using the additional swap file. Is it possible the change you're seeing now is actually a result of previously lowering the swappiness parameter - and it just needed a reboot or something to finally take effect? Because the change you're seeing now is pretty much what I would have expected to see from just lowering the swappiness.
Discussion
I haven't rebooted. I added a swap file with higher priority instead of making the original one larger precisely to avoid rebooting. I don't dare interrupt Bitcoin Core.
Understood. It seems entirely likely to me that without rebooting, the alterations you are making to the swap environment aren't even actually applied yet (until reboot that is). The growth in RAM usage could simply be a factor of Core's memory requirements growing as it progresses through processing its history of transactions.
At this point, it's all voodoo, mate 😂