Thank you, glad you like it.
No, it doesn't have to be the i3 mini controller. I've picked this one because of its small form factor and because it has all components needed on one PCB.
You can use any other controller board based on the ATmega2560 chip but you'd have to adopt the pin configuration and the housing yourself.
Yes, it's pricey - no doubt - but I guess that is to be expected when geeky freaks build something for their own day to day use and simply expect them to function in every single print ;o)
That's also the main reason why my printers are all equipped with (genuine) Duet3D boards.
Don't forget, that if you build such a machine, you'll get a five fold Bondtech extruder equivalent, which doesn't come cheap even if you get them from aliexpress (which I'd not recommend - intellectual property - yadda yadda).