Cleaning the old map

I apologize in advance for any mistakes, English is not my native language.

This summer my friends and I started the server with 70+ mods (1.12.2, server is on hosting), during the time we played we realized, that it takes too long time to launch both the server and the client, plus some mods turned out to be useless. We deleted some mods, continued to play. Now our modpack contains 55 mods, everything is okay expect connecting to the server. 4 out of 5 connection attempts fail (client just stops responding, after that we need to reboot it). It takes half an hour to connect to the server because we reboot the minecraft several times. My goal now is to solve this trouble.

I have already read some forums and I think that the problem is in saving. It is old, and as I understand, now it contains a lot of junk information and information from recently deleted mods. That's why I want to clean it up somehow.
 

1)First attempt was to copy WorldName\region folder + level.dat and some others to the new world saving. As I expected, nothing changed. 

2)Secondly I tried to create the new one world and to copy all structures using WorldEdit schematico (used different formats of saving: schematico, mce, mcedit). The structures was copied, but with completely different blocks.
2.1) I used NBTExplorer to look into the level.dat file. It turned out that IDs in minecraft are initialized dynamically and are unique for each world with different modpack. Here it is the id of bell block in old world

Here is the id of the block in new one;

2.2) I copied the level.dat file to a new map and tried to use WorldEdit again. The results are: some blocks are copied true, some are not. 

From this point I don't know what to do. I have already read all local Russian-speaking forums looking for a similar problem: without any results.

That's why I'm here. We spent a lot of time building beautiful city, and I'm afraid that the situation will only get worse. 

Is it possible to clean up the world somehow? 
If not, is it possible to copy out the structures from the old map with IDs initialized this way? 

Maybe the world is not even the cause of the connection problems?