Hi, I have a bit of an odd one, and not necessarily an issue.
I'm running the same version of Tectonic on 1.20.1, but generating new chunks somehow causes a noticeable line in height between new and old terrain, and I have seemingly changed the config at some point of the modpack existing (BOP, Terrablender, Regions Unexplored & Tectonic versions stayed the same though). Is it possible to figure out the numbers I used in horizontal_mountain_scale and terrain_scale? Retracing my steps returned only broken worldgen, even though I found the client configs for Tectonic.
I need to regen chunks because of new ores but I do not want to destroy the look of the world, so this is my only hope left (other than bruteforcing, which will take literal MONTHS)
Thanks.

Example pic:

How it should be:

Here's an egregious example of that, I don't know why, but the terrain generates too low.

It seems that structures however think the region should be higher.

I also have Noisium, but I use the same version. Strangely enough generating the entire world from scratch on an old version of the modpack (for which I have found the data - 0.35 horizontal / increased height true / vertical scale 1.0) then upgrading and keeping the world in singleplayer makes chunks blend well, but using it in multiplayer on 1 region in the middle of the map causes it to not blend at all.

Check the old files of modpack if you have them.
Other than that, you just need to test it with different values under "experimental" in config.

Check the old files of modpack if you have them. Other than that, you just need to test it with different values under "experimental" in config.

I found them, if I generate the world using the same seed, it's OK, but if i remove chunks from the existing world on the server in order to regen it doesn't blend properly.

What are some good values? I've tried the extended height being true, which works, but I raise the horizontal terrain value and instead of mountains being more gradual they are almost straight up.

What are some good values? I've tried the extended height being true, which works, but I raise the horizontal terrain value and instead of mountains being more gradual they are almost straight up.

It's in the comments, lower values mean more wide and progressively high mountains.

I found them, if I generate the world using the same seed, it's OK, but if i remove chunks from the existing world on the server in order to regen it doesn't blend properly.

Blending isn't enabled on existing worlds, you have to use a world editor like MCA Selector in order to force blending on.

To answer the original question, there's not much you can do other than trial and error.