Starting to design out my build, looking for critique

[Chrono]

A few comments I would have from your last set of design drawings are:
- whilst 4 rails looks like a good idea, you will have a devil of a time aligning them I expect. Maybe 2 rails of a larger size might be better?

Also, on the four rails idea, I think you'll definitely end up spending a few days becoming best mates with your dial gauge - but it could pay off in the end?

It seems I have messed up my explanation here. I do not plan on using four rails, but instead three. Two linear HIWIN-style rails on one side, mounted atop each other. The other side only has one rail, and a different type, namely the cylindrical Igus-Style rails that you also often find in 3D printers, like the Prusa MK3. That will still take some work aligning, I get that, but it should technically work to help let the machine compensate. I know, technically is a nice word, but practically often takes precedent :D I have added another picture, with the HIWIN rails in their place, to hopefully make that more clear.

[*] 10mm of steel looks quite beefy for a side plate, but 3mm seems quite weak in my head. I realise its intentional, I'm just not convinced!

This one's along the same vein, and it honestly also seems off in my head, but the math should check out there. Ultimately, the sideplates don't cost much to make, especially not the smaller one on the left, so if it turns out to be a problem, I can change it around without much of a problem.

First impressions on the redesign; I think your CAD is coming along nicely, really shows. I do have a few concerns though:
[LIST][*] I question the Z axis travel length and why the plate is so large - Don't forget you'll have a bit attached to the end of your spindle, and its going to have to be quite a long bit to make use of your Z travel. I've got 18cm, and honestly, its probably far too much, most of the time you're really only needing to cut say 18mm plywood! The distance between the bed and the gantry looks huge - All that Z extension reduces the rigidity of the machine. Have a think about what you really need - can it be acomplished in other ways, such as an adjustable height bed or overhang section which keeps everything stiff for the majority of the time, but when you need to use the router on something large you can?

Something like that came up when talking to my colleagues as well, today. We came up with inserting angles to hold up the sides, which could be removed if needed, but when there, would make it a bit stiffer. I really quickly added one in the picture above, highlighted in pink. Imagine four of those in the corners.
Could you maybe elaborate on how an adjustable Height bed and that "overhang" would work? I have a hard time imagining those two.

[*] How are you going to assemble the Z axis? Look through the design, building it up step by step, and imaging when you will or won't have access to each screw and what that implies. I've got my ballscrew and motor on the fixed bit because I can easily get to the four screws holding the ballnut onto the Z plate, undo them, then slide off the rails. Your design looks a little harder to physically assemble.

Good shout! On the first look, I should be able to do a similar thing, but with removing the screws of the Z-Ballscrew Bearing Blocks instead, but I will definitely check that again.

[*] The C beam - how are you going to attach those profiles to each other? Any weakness here will be quite problematic and you've got two joints which need to be constrained.

I have enough space between the screw and the profiles, to be able to insert some angles to bolt them together (that is going to be some fun fiddling with angled hex drivers, however :D) and I can also bolt them together on the back side as well. With the sled in front, forming a closed frame and additional bolts in the side-plates, that should be enough then, right?

- Also consider the CS-Labs kit for the controller - many on here use them, including the venerable JazzCNC (which is always a good sign imho)

I second Washout's comments about the CS labs stuff, looks brilliant. The thing that put me off was that the base model is only 4-axis, and it doesn't handle slave axis homing native-ly (although there are decent workarounds posted on here). Anything more than the base model is horrendously expensive, and I wanted future expand-ability to add a 4th axis which is gone if you use the 4th for a slave - but you avoid both of those issues by having a connecting belt and single motor.

With that much of a recommendation, I will definitely take another look. But I must admit, the price of the units, especially compared to the other controllers is quite steep. I might just stick with one of the cheaper ones and maybe upgrade later on. I am managing to stick within my initially laid out budget for now, but I want to also reserve a buffer in that, so that a problem won't throw me off course completely.

- Proximity switches if they are the type in a threaded housing, will give a nice amount of adjustment for the trigger point - you can do the same with micro-switches, but they then need a plate with slots to mount them to to achieve that

Yeah, that was my thought. My 3D printer uses a proximity switch for leveling, and needs exactly that adjustment, which also means that it can be calibrated pretty finely.

I'm still aware I haven't gone through your calcs yet... Finding time is hard! Sounds like with the design changes though I may wait a little longer :)

Don't worry, no rush :D I am pretty sure I have suitable Y- and Z- Motors, but the X-Stepper is what I am still fiddling with the spreadsheet for. My main problem is how to calculate the thing driving two ballscrews at the same time. I just added a multiplication by 2 in the part of the equation that contains the Screw Inertia.