Hi there,
I recently wanted to build something similar, a loop selector. Like the MXR M-144, but better
I found the website "General Guitar Gadgets". They have a shop, too, but they also have lots of schematics/instructions on how to wire up all sorts of switchers. The one I used for the loop selector was
this one (link to PDF).
Maybe just browse their website to see if there's anything that suits your appetite. Here's some links for you:
https://generalguitargadgets.com/effects-projects/boosters/effects-loop-switch-boxes/
https://generalguitargadgets.com/effects-projects/boosters/effects-order-switcher/
As far as my loop selector goes I like that the pedal grounds the "Send" of the unused loop. I take it that's responsible for the pedal not making popping sounds when switching between the two loops. So there's no need to add any pull down resistors.
I use it to switch between my clean sound and my dirty sound with only one button. I love it. I basically followed GGG, but I didn't add the LEDs. I don't care for LEDs on this pedal and this way I can get away with only a 3PDT instead a 4PDT. And I didn't connect the grounds from the left side (Output, Return 1 and 2) to the right side (Input, Send 1 and 2) because I'm thinking it's not needed. Anyway, it works like a charm.