There's no one answer, but there are many options and a lot depends on you. Let's cover some details that may add nuance:
1.) What Mac Mini is this, and what are you doing with it? The issue of suboptimal size and resolution and how the Mac scales may put a little burden on your graphics processing system, but I question whether it's likely to make a notable impact.
2.) How much of a stickler are you for text sharpness? Are you a nitpicker with an eye for fine detail, or an average Joe? Is the difference between 4K and 5K likely to be a deal breaker?
3.) Do you operate in a high glare environment where matte finish is desired, or are you one of the glossy mainstream?
4.) Is 27" fine, or do you want a 32"?
5.) Is this monitor doing to be used for anything else, like hooking to a gaming console or PC, such that refresh rates higher than 60-Hz (e.g.: 120+) are desired?
6.) Does it matter whether it has built-in speakers or webcam and how good they are?
7.) Does it need to offer USB-C (or Thunderbolt) connectivity and hub function?
8.) How much money are you willing to spend?
Without the details, we're kind of shooting in the dark.
This YouTuber has a video advocating for a nice Philips up around $800 or so?
But if your needs are more basic, this (also a Philips) 27" 4K monitor with USB-C connectivity and hub function is on sale around $300 or so. I picked one up in 2021, and while it's not my main monitor since I'm on a 2017 27" iMac, it seems fine when I use notebook computers with it.
PHILIPS BRILLIANCE 279P1 27" FRAMELESS MONITOR. MASTER EVERY TASKThis Philips Brilliance USB-C monitor replace cable clutter. View UltraClear 4K UHD and re-charge a laptop, all at the same time with a single USB-C cable. SmartErgo base, Flicker-free and LowBlue mode make work easy-on-the eyes.ULT...
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