At what speed does the RTH occur. The most recently used or Cine, Normal or Sport?

Nominally none of those, if you look on page 15 of the manual, in the "RTH Procedure (Straight Line)" section, the manual states 10.5m/s. N mode speed limit is listed as 10m/s.

The above said, that relates to an entirely automated RTH with favourable winds and NO input from the pilot. Unfavourable winds can reduce that speed.
It also seems the pilot can increase that speed in some circumstances by giving forward stick on the controller. I wish to test those circumstances myself but it has been too windy here of late.
It may also be possible to reduced the RTH speed by giving reverse stick, I have certainly done that with a Mavic Mini but I don't think I have tried it with a mini 2. If you are inclined to do so make sure you are experimenting in a safe place.

Hi, Startup. Thank you for reaching out. Kindly refer to Sean-bumble-bee's reply above. To add, you may see the manual on this link: https://www.dji.com/downloads/products/mini-2. Thank you for your valued support.