It seems to me the primary load that these parts have to support is to hold the X rails perpendicular to the Z rails. In other words, the purple and red pieces must avoid twisting relative to each other.
The X tubes are not under any axial load, so it doesn’t require a lot to keep them from slipping axially. Covering the ends with a lip means that the tubes need to be exactly the right length (or a bit short) to get the wheels to track parallel. Since they are not under any load I’m not sure of the advantage.
Z tubes have a little bit of axial load and the opposite end is free, so I think a lip makes more sense there.
The X tubes need to support being pushed horizontally, which is naturally going to be stiff. Pushing the X tubes vertically is necessary for a stiff Z axis and also to prevent tipping in the Y-Z plane. For a stiff Z axis the transmission of the load from the X tubes to the Z lead screw needs to be decent, and for tipping it needs to transmit the load from vertical load on the X tubes to vertical load on the Z tubes, which is ultimately supported by the plate. These are relatively easy to make stiff.
The point is that the different modes of deflection are very uneven in their importance. I might not have identified the deflections correctly but I do think they are unequal. Maybe the green and yellow parts could be replaced by zip-ties and the red/purple parts deserve all the focus.
As for being adjustable or modular, I am still unclear on one thing, which is whether the tube spacing is going to remain the same so the other parts remain compatible (given their own adjustment) or do you have families of parts that go together and possibly a size-dependent plate?