You didn't mention the game but could guess it's CS:GO, if it is...
The process is much more simple then you're making, you don't need to "cross my fingers and hope for the best", simply follow the instructions CS:GO gives you. If you're playing on Casual (game_type 0 & game_mode 0), you create the gamemode_casual_server.cfg. This is where you put all your mp_* values you want. Same process if you're using competitive, or whichever game_mode + game_type combo.
https://developer.valvesoftware.com/...ual_server.cfg
Your server.cfg is correct and really that's you should be having in there, not any values of mp_*. You don't add in game_type or game_mode into your server.cfg, you should have that in your start parameters when launching the server.
Use gamemode_servers.txt to add in your mapgroup if you are NOT using a workshop collection. Since you ARE using a workshop collection, add it to your server start parameters.
Code:
+host_workshop_collection 1337 +workshop_start_map b3475
Where 1337, you add your workshop collection
Where b3475, you add the workshop map you want to start with