It's possible to mod 735 Trinitycore without a doubt, it's just not easy-easy. I'll give some hints and help for the Windows POV.
Few things you need to figure out:
1. Right VisualStudio version to work with (I'm using 2017, which was up2date when 735 TC was released)
2. Right boost version to use (I'm using 1.64.0 with, and this is important, lib64-msvc)
3. Blizzard is a d*ck, so you prolly need your own CDN to host the normal client if you don't want to have people download the whole client at once
4. CASC is the key to success now, this is (since there is no good modded executable anymore) your only way to get custom files into the client. Remember that you need to put the .build.info in your clients folder after the CASC is done. You can find it in the output folder of your CASC.
5. ADTs and DB2s are a little bit of a hazzle, there are some ADT converters which will work, some others might not. For DB2s, if you don't want to do things like creating items, the only real thing to remember is to always put whatever you change on serverside also on clientside.
As someone working actively on a 735 server for like something over a year I'll happily answer questions if there are any.
And the error OP encountered happens quite normally when the CASC has (or was) screwed up or when the client can't fetch the update for some reason(e.g. blocked by a firewall)