Mac mini M2 with LG 34WK95U don't recognise 3440x1440 as HiDPI

What cable connection are you currently using?

USB-C, HDMI or DisplayPort

I would suggest using a USB-C to DisplayPort 8k cable.

for example > https://www.amazon.com/dp/B09M38CXZ3/ref=sspa_dk_detail_0?

Then to somewhat address your HiDPI question, have a look at the following link.

https://knasmueller.net/scaled-hidpi-mode-on-an-lg-ultrawide-with-a-16-mbp-2019

Mac's and LG's don't play well on a Thunderbolt/USB-C cable.

HDMI is limited to 1080p even using a short premium certified cable.

USB-C to DisplayPort will give widescreen resolution options at higher refresh rates.

I just used the Sikland for example. There are dozens of other USB-C to DisplayPort 4k and 8k cables, that will work just fine between the Mac mini and that LG display.