You probably know most of it so just some advice: Don't format the Partition table (MBR to GPT etc.) on the disk whose data you wish to keep.
Shrinking a partition or moving it carries a small risk of data loss and will take significantly longer than creating a new partition (since data needs to be cut and pasted from one area of the disk to another). If your old laptop has an empty slot for another SSD or NVME drive you can plug that in, and still dualboot and having the new drive Linux only.
Also to deal with the occasional Windows cockups, just carry a boot-repair USB, the auto repair has fixed the Windows issue for me 90% of the time (the other times are usually boot order priority or other BIOS setting)
The feel of Lemmy communities is a little different than Reddit, even if the software features are mostly analogous and there are many Redditisms used.
Your average commentor/poster will stand out more in a small community, there's less of being able to post and then slink away.
People have gotten used to a lot more comforting features of modern Reddit, Lemmy in both the users and in the software has more of a "Reddit 10-15 years ago" feel to it.