Hi @GC67 I agree with what @benowe1717 said: all three paths can work, and the best one really depends on what you want out of the experience.
Raspberry Pi 500+ is a great low-stakes way to experiment. If you end up breaking something, it’s easy to reflash and keep learning. But for heavier dev work, especially Java, the CPU might feel limiting despite the RAM. It would shine as a dedicated box for your smart-home project though.
ThinkPad T480 is one of the best bang-for-buck options for Linux. Excellent compatibility, easy to upgrade, and solid performance. If you end up loving Linux, this could become your daily driver without much hassle.
Dual-booting your Dell is the easiest starting point. With 16 GB of RAM and a 1 TB drive, you can comfortably allocate 200–300 GB to Linux and keep Windows around. It’s a great way to learn without committing to new hardware!
You’re definitely heading down a fun rabbit hole. 
