There is a significant difference SD44 has in terms of defensive options which WG largely did not have to deal with and it's stationary AT-guns. To summarize, AT guns feel like a separate tower defense mini-game in a game mode where fast combat and zerg-micro abruptly meet leading to friction. If you manage to deploy a number of AT guns you either achieve herculean feats to stymie a platoon of vehicles or have stuff on the field that is easily countered by air strikes or indirect fire that result in negative returns and a futile exercise in throwing away good points after bad. Which ever way you look at it, AT-guns don't naturally flow with a meeting engagement core game mode and feel lopsided leading to some player frustration.
I think the Phase concept was a good idea *if* we wanted players to establish a front-line where it would make sense to set-up strong points with AT-guns while trying to probe the enemy for weak spots. Then, Phases B and C would then turn to a game of assaults mimicking some elements of real life while maintaining the semblance of a fun and fair game between players with the pace being throttled up. In order for something like that to be functional a lot of typically Phase B units would not have been available in A and thus lose some coolness factor. However, to do so with a meeting engagement scenario feels like the game is forcing players to play a certain way rather than let the options provided by the player organically develop to make logical decisions. There was the coop mode which adequately did the front-line mechanic but this didn't solve the issue of why "defense" felt different in SD44 than compared to WG.
Also, most Anti-Armor options available are filled with expensive and vulnerable AT-guns. There is a real struggle with AT Guns as they do emulate the correct historical drawbacks which significantly impacts AT gun usefulness when we get to actual combat which is quite fast and feel lopsided in either direction. This wasn't so much of a problem in the WarGame series as most Anti-Armor options you had were very mobile light units (armored/unarmored vehicles or helos) or infantry with long range ATGMs which benefited by being entrenched in buildings like other infantry. Being historically accurate, AT-guns generally are not a good stand-in for the mobile type of combat SD44 came from with a WG game mode.
That being said, I'm not sure what improvements the devs have in mind for SD:2 to make things different and less complicated for new players. I can see why the OP would make a suggestion to have bunkers though I don't think that would work with a meeting engagement scenario. If the goal is for players to set up a frontline or initial defense backed up by stationary strong points then that best should be served with a better designed game mode. There is less of a struggle with WG series and to me one of the obvious things is that mechanically AT-guns do not really flow as well as it should with the core game mode like it did with ATGMs.