There is no denying that Mayhem 2 had a few additional features which got cut for 3. I can understand where you're coming from. In the end its just a game and you could also go back or play something else like improved races mod.
From my POV all the things you mentioned like defense platforms or Boarding are only 'gimmick' features. They can be fun and might often seem special or cool. But they don't really fit into the general concept of the mod or even go against it. Big problem is that only the player can effectively use most of them and they feel disconnected to the core gameplay loop. Its basically minigames.
The ultimate vision for Mayhem has always been this sandbox where the player personally builds his empire on a small scale, so that his actions actually matter. You are not just the fleet commander like in LU, you are the warchief who can make a difference in the frontline or by helping the industry at home. The AI was also supposed to play a bit more like a player and fight for territory.
This is the Mayhem concept in a nutshell. Naturally the mod slowly went more and more into this direction to create a single coherent game experience. Streamlining and cutting that which didn't fit in. Removing or replacing features due to reported bugs or problems. Not everything was perfectly planned and certain things got cut because something didn't work. Best example are the Terrans and their hardcoded relation with the ATF, so they got removed. I think there have been massive improvements with each Mayhem iteration.
Mayhem 1 only had the 1st basic version of the turf war but without any more content which was tailored for the new game environment.
Mayhem 2 covered this part and added every vanilla X feature in existence in some altered form to get lots of gameplay content. By doing that, here and there it strayed a bit away from the base concept in my opinion. When i read about tactics which are basically exploits like the defense platforms in front of gates i don't get where the fun is with these. Sure its necessary because of the OCV that only attacks the player. But the idea was that player and NPC are not much different, no?
Mayhem 3 mostly wanted to get rid of the jumpdrive and add NPC ship building with real resources.
To me the removal of the jumpdrive is a feature. Suddenly you have undefended sectors where the enemy also cannot bring forces in quickly. So concepts like 'attack' or 'defense' got a real meaning for the very first time. You cannot jump everything around so you need multiple fleets.
Additionally all these little stories unfold, when you just hop in a freighter and personally buy a load of Ore to finish building an important factory, evading pirates on the way home. Unlike LU, here your personal actions actually matter because that one load of Ore actually gets a lot of things done and matters a lot for your further progress. In any other mod it would just be another load of Ore that you don't even need because you buy stations with credits. So everyone can play his favourite gimmick features to grind credits and that gets called empire building.
But Mayhem has actual hands on empire building. Its one coherent game loop.