There are lots of 4K monitors out there. The image on a 27" 4K monitor isn't quite as crisp as on a 27" 5K iMac or a 27" 5K Apple Studio Display, but given the way that Apple chose to price the Studio Display ($1600+), a lot of people have chosen to go with 4K monitors instead. 4K is enough to let you use Retina scaling modes, and with modern versions of macOS, a 27" 4K monitor running in Retina "like 2560x1440" mode may have higher-quality text than 27" 2560x1440 monitor running at native resolution.
You can get a 27" 4K monitor with an IPS panel, near-100% coverage of sRGB, modern inputs (DisplayPort, HDMI), and a height-adjustable stand for as little as $300 – $350. Most monitors from vendors other than Apple use VESA mounts, which give you a standard way of using another stand, or putting the monitor on the end of an jointed arm, should you ever want to do so.
Spending more may get you features like
- A USB-C (DisplayPort) input that lets you hook up video, hook up hub ports on the monitor, and charge your MBP using a single cable – essentially turning the display into a mini-docking station.
- Near-100% coverage of wide gamuts (DCI-P3 and/or Adobe RGB)
- Advanced color accuracy features like the ability to store calibration profiles in hardware. (This would be mostly of interest to advanced amateur photographers and professional photographers.)
Some monitors may have the ability to swivel to portrait orientation (where the display is taller than it is wide).