Hard to say, I would start with a macropad. There are a lot of project out there so you can browse around until you find something you like.
Here is a recent one: https://hackaday.com/2024/10/21/zero-to-custom-macropad-in-37-easy-steps/
It requires quite a bit of skills and tools, although jlcpcb can solder on the components for you so you can focus on only designing if you wish.
I would use Kicad to make the schematic and pcb, and Fusion 360 for any 3d modeling. Fusion has a free non commercial version, you just have to find it. They kind of hide it on their main website.
I like phils lab on YouTube for pcb design stuff but he isn’t exactly beginner friendly and mainly focuses on audio related things.
I respect him for it but I really wish he would surround himself with a competent dev team, at least for the new one.