Core 2 Duos are slow, yeah. I've got an Asus F8SP-X1 laptop from ~ 2008 with a Core 2 Duo T9500, 4 GB RAM, and a SSD SATA drive in it. It was originally a mid-range Windows Vista system. Over its years I managed to upgrade it as far as it could go. It does run standard Ubuntu and Windows 10 - Certainly not fast but it does run. Performance would lean towards unbearable without the SSD. I suspect Gnome isn't doing it any favors and switching to a lighter DE or distro would help (or maybe just ditching the DE altogether) but since it's just a spare laptop it's no big deal.
One of the takeaways from your experiment is if it the system was already crap at running Windows 10 it's not necessarily going to fare better with Linux, at least if you're expecting a nice desktop environment. I don't know if in 2025 we need to equate the "will this run Linux?" challenge on old Windows XP/7 hardware aside from the geek/techie users that want to do something with that old hardware. Anyone else non-technical stuck with that type of hardware isn't thinking about Windows 10 being retired.
That should work fine.. I suspect that failed maybe because you renamed like you said. Make sure Transmission is adding torrents in paused mode, then do another test with a torrent you definitely didn't rename. Maybe just do a test download in qBittorrent and then attempt to add it into Transmission e.g. a Linux Mint torrent or similar is usually a safe test https://www.linuxmint.com/edition.php?id=319
Because of how you have your torrents organized it does sound like you'll need to tough it out and add each torrent and configure it manually.
It would be easier if you had all the torrent data saved in the same folder(s), in which case just configure Transmission to add torrents in pause mode, configure a watch folder, copy your qBittorrent's .torrent files into that watch folder, and finally do a re-check in Transmission and start all the torrents. Then just hardlink the torrent data out into your own nested folders how you want them set up, that way the same data exists and is linked in two places (torrent data folder and your own folders). Maybe it's something to consider for your future configuration but it's not going to help you much right now.
For now yeah, the best you could do is set Transmission to add torrents in paused mode, configure a watch folder, copy paste your current qBittorrent .torrent files, then afterwards in Transmission change each torrent's data location and re-check one-by-one. Not sure if it's any faster than just adding the torrents manually one-by-one :/
You should be able to find the current .torrent files wherever MacOS saves your qBittorrent files, look for a folder that looks like qBittorrent / BT_backup, all the .torrent files in BT_backup are your loaded torrents inside qBittorrent.
With some luck maybe you can find a tool that does qBittorrent --> Transmission migrations? I wasn't sure if any exist, all I can find are tools to do Transmission --> qBittorrent e.g. https://github.com/undertheironbridge/transmission2qbt
(note I'm not on MacOS so maybe someone else has more direct advice to offer)