unless you really, really, really, need dedicated F-keys, i'd suggest the Quefrency
the biggest thing with custom keyboards is programability, and with that comes tons of alternate/unique layouts.
- if you're just getting into it, 60-65% is a great starting point.
here's just one generic example (this is a pretty normal-ish 60% and how we make use of these smaller keyboards)
- hold the key marked "MO 1" momentary, layer 1, and you activate Layer 1, all of the F-keys are there
• hold "MO 1" and tap any of the number keys to get the corresponding F-key.
• this is probably one of the most common layer/Fn functions and is super easy to use & get used to.
- tap the key marked "TG 2" toggle, layer 2, and you turn on/off arrows instead of the default mods & shift in the bottom right.
again, this is just an example. because these are programmable, you can make they keymap whatever you like.
if you want a more complex example, look up the default keymap for the plank (40%)
figure out what you can & can't live without and use that as your guide for picking a keyboard to build