Well my map has access to all cities and areas of interest that do not require going over any mountain, sure I have massive mountains but they are not necessary to climb unless I have a quest somewhere up one. I have had one quest which did require going up a hill I could barley go 1 KPH or less on a minibike I think I would have been better walking up but that was the only one. If you look at my map, there is a lost of paths you can take to any city without going near a large mountain.
If you do decide to do a Nitrogen map, do yourself a favor and do not add "vicious cracks" (they are massive and when you get stuck in them you HAVE to walk out there is no way any vehicle can get out of them) and chose "many roads" and "smoothest" for smoothing, I chose smooth and got a lot of terrain that is just bumps and small hills but the roads are damn near level everywhere.
Compared to Navegane the mountains are ridiculously high but there are few of them with the "smooth" option and I am sure smoothest" would have been even better. I am just happy with my map and maybe you have had bad luck, but I will never go back to Navezgane as it would be just too boring.
I suggest to keep at Nitrogen with the options I have mentioned and you will get a good map eventually. Do you have a PC that is taking too long for a 8K map? I get one in less than ten minutes on a medium PC with 16GB Ram and a 3.3Ghz i7. Also If you have a good image editor you can smooth out the height map by using a simple blur filter or just blurring the edges. I could do that for you if you would like, just save 8K maps in the options and you should have an image that is large enough to edit. The smaller the image the less editable they can be, an 8K map is 8,192 x 8,192 and is easy to edit with an image editor, that is if you have more than 4GB system ram otherwise you might be using your page file when editing. I am not that sure about it but for an image that size you should have well over 4GB on a 64 bit PC.
Heightmap Original:
https://i.imgur.com/lnD8i7J.jpg
Heightmap Somewhat Edited:
https://i.imgur.com/ooqDxTJ.jpg
Heightmap Heavily Edited :
https://i.imgur.com/IkHfZSU.jpg
Not too sure how this will impact the map and it would be a bit boring to just have rolling hills but if that is what you wanted this would probably be the way to do it. It took about 5 minutes to edit with a Gaussian Blur and some basic filters.
The POI HM image has to be the same, just save as genHM and poiHM. The preview map would not be any different as it only shows the traders and city details. Like I said if you upload your genHM.png I could smooth it out for you and re-upload it too you and then you can get in the map and see if it works this way. To be honest I have no idea if this would work or not but it is worth a try.