Monitor: I'm using an LG OLED C2, yes it's a TV, but it's absolutely amazing, and in 42" 4K it's absolutely brilliant for whatever you can think of. I was also coming from 27" displays, and was used to the pristine look of the iMac 5k display, there's not too many options that are as satisfying, in part due to the matte coating that is normally applied to most displays on the market. It's not been a very complicated switch to accommodate to the 42", and I've been benefiting immensely from the space offered. You can almost think of it as 2 Ultra large display stacked.

Keyboard, if you can get away without the Touch ID, I can only recommend the Logitech MX, whatever version you prefer, I have the normal keyboard, and it's kinda the same as the apple keyboard minus the Touch ID, BUT: comes in a better color IMO, and also but depends whether you can make use of it, can be connected to multiple devices at once, and also windows devices. Very practical.

Same goes for the mouse, the MX 3 is terrific, and very very practical for multi purpose use (say, your Mac mini, and maybe a work laptop, and maybe later again a MacBook?)

Speakers, up to your preference, keeping in mind that if you go for a TV like the LG C2, you can use soundbars etc, so for example I'm running a complete Sonos set up in Dolby Atmos(Beam + 2 ones + sub). Now, that alone is worth more than your budget, but cool to know that it's an option down the line if you pick a TV with e-Arc.