Starting to design out my build, looking for critique

[JAZZCNC]

Understood. I've been scouring the forum and the web, but it seems my search-fu is a bit off, since I could not really find a conclusive answer on the differences of the UC300eth, UC400eth and AXBB-E, especially since CNCDrive's web pages are kind of scarce on the matter. As far as I am able to see, the 300 just has more IO than the 400, withe the AXBB-E has the same amount of IO as the 400, but a built in Breakout in a nicer package? Judging from this post, it seems that I can use all the IO that I can get, which would tip me in favour of the UC300eth.

Also, you mentioned being locked into Mach 3/4, as a negative. What other (better?) alternatives are there, and why is that?

Yes your spot on with differences between these boards. The UC300 is the better option if you need lots of I/O. Coupled with a Good Bob that can accept 24v signals it's an excellent setup.

The alternative to Mach3/4 is UCCNC. Whether it's better or not is matter of opinion. The trajectory planner or put in simpler terms the way it creates motion is better and smoother than Mach3, can't speak for Mach4 because it's so full of bugs don't trust it and won't use it.
Now mach3 as it's advantages in that it's got massive following and is much easier to write custom scripts etc and customise to your exact needs.
What can say is both work well and will do what 99% of people want straight out the box.

I guess I just ignored the existence of 2020 Ballscrews :D I calculated it through in the motor spreadsheet, and, asuming I did all the correct modifications in terms of doubled motor RPM and doubled torque, I could actually now hit 12m/s rapids with the same motor, compared to 7.5m/s before. Thank you for the recommendation!

If by12m/s your meaning 12 meters per second then your massively off with your settings. 12 Meters per Min is more realistic figure.

To be honest that spread sheet is load of rubbish and spits out unrealistic figures. Real world is much different due to the many variables that affect machine performance.

IBuilding a PSU should be fine, for the most part, but just so I can make myself a complete picture, do you have any recommendations of AC Drivers that are worth it?

If you can find any the AM882-H are AC version. They are hard to find now thou.
If your on a budget these are not bad drives which i've fitted on several machines without any troubles and good performance.
https://www.ebay.co.uk/itm/2DM860H-2....c100148.m2813

I mostly tend to fit Closed loop hybrid steppers these days and many of them can be driven with AC or DC.

On a different note, I have seen Jog and MPG pendants mentioned and presented on shop pages. What do those things do exactly? What I have found repeatedly on the topic, is that they are used for manual operation of the machine, however I was under the impression that this would also be possible directly from the control software. Am I mistaken here, or is there a big advantage to using such pendants?

MPG / pendant is just a hand held box that lets you stand at the machine and Jog it arround rather than using the PC keyboard. On large machine this is very helpfull when setting tools or find edges of material etc as walking back to PC to make move is a pain.