My first synthesiser DIY build was a Shruthi, a now discontinued desktop synthesiser project from Mutable Instruments. It wasnāt a kit, I got the PCBs from a group buy and sourced the components myself. I already had quite a lot of soldering and electronics experience. It is possible to get kits from tubeohm in Germany, which I have heard good things about. They rebranded it and call it the Phoenix. If youāre in the USA, then I think it is possible to get full kits from Laurentide synthworks.
Itās quite a bit bigger than a Bastl Kastle, but very easy to put together (all through-hole components), and much deeper and more flexible than it looks at first glance!
Good luck with DIY and soldering! Great skills to learn, and loads of fun.