Probably possible to build a custom around rm300 bucks

i) handwire switches (about rm58 for 110 outemu white switches, diodes are dirt cheap)
ii) laser cut acrylic full size plate (less than rm100? from shopee sellers)

rest of the money goes into the controller..which I'm not aware of any off-the-shelf KB controllers with 21 IO off in a kit. maybe Frankenstein donutboard with a compatible MCU?

for that budget, you can try this one, https://item.taobao.com/item.htm?id=536060210170

however, be warned that there are risks getting these budget kits.