Hi Sweet - I'll try to work through your thoughts:
1) thinking is not dangerous. But once you have critically worked through the issues and decided a trajectory then its time to leap into action
2) Find a commercial machine that does what you want with good support. Commercial support has real value. Then talk to your bank or whatever and figure out the terms. I understand you don't like finance but that's how businesses grow. You have to leverage time and money. If you build your own machine I expect its a year or more before you have it earning, in that year you can earn alot with a new machine. At least go thru the exercise so you can compare your build costs. Commercial work and an ATC makes sense it saves huge amounts of time. ATC is not easy for a hobby builder to sort, again a commercial ATC machine is what you are aiming at....
3) 15mm rails will work but I use 20mm and have recently decided to go to 30mm for various reasons. Rail and rack come in sections that are matched. You need to work with a supplier that can grind match the ends. Don't need a "joiner" if matched ends are done. if you go helix rack (which will be right hand tooth) then you also need to order a short length of left hand rack to use as an alignment tool.
4) N23 or N24 size motors are fine with a 5:1 box
5) Do not spend money on hybrid steppers, I don't think the extra dollars is worth the small gain. If your balancing a build budget it will be std steppers or servos. Servos lift the game to the high level.
6) I would not use steel as the reinforcing for ply. I'd use aluminium. In this way you can use trimmers and routers to finish parts. Steel presents issues unless you can work with steel in total
7) Do not go down the steel welded path. If you weld it distorts and unless you have access to heat treat and finish machining then it becomes a mission, especially on a large machine as you need a very large mill to machine the lands
8) If you have a good sheetmetal shop that can bend accurately then look at folded sheet metal parts. These are accurate within the parameters you need. I build kit routers like this. Half sheet machines. This is a very valid approach. As with all technologies there is a learning curve. Its taken me 4 machines to learn all about sheet metal builds. Scoot my production half sheet kit is 90% sheet metal parts.
9) I recommend not to use std hollow sections they make you design non optimal parts. They are not flat or square and have cupped or crowned faces. All of which has to be fixed along the way
10) Sounds odd but having designed many many machines I start at sorting the Z axis first. Design a great Z axis that does what you want and work out from there. If you start elsewhere by the time you get to the Z you inevitably are squeezed for space and dimensions. The you have to expand all the rest of the machine when you find that the Z is too big or you compromise the Z and then you've cooked your goose. The Z axis is the area that does the work and unless thats uber stiff and sorted then the rest is just along for the rid.
11) You can build a totally satisfactory plywood machine. There is a german company that does this for cabinetry, and they do a great job. If you build stay in a medium you understand. If its a hobby fine, experiment but to make money you need to be brutal about the exercise. Your size machine is an $8k to $10kAUD exercise for me with no cost for my labour. Parts , freight etc add up. I've just built a bench for a 1/3 sheet 5 axis machine and its costing $800AUD for the materials and so it goes on...
12) In terms of control you need something that can scale ie add axes or controls easily. Look at Dynomotion controllers. Took me a couple of years to find these guys. I've used a few different controllers and systems and I think Dynomotion has a lot going for it...

CNC Wood Milling Machine | Mechanical Engineering | Am Luftschacht 3, Gelsenkirchen, Germany (cnc-holzfraese.de)

12) Do not buy anything until you have designed it. Its fine to source and cost stuff but buying too early will result in things sitting on selves... If you have stuff that maybe useful muster them into a single place and see how they go. I set up a folding table and start mustering parts and stuff on that as a project table. If its in sight its more likely to be used and if you see things often your subconscious will work on it more often.

Keep at it... Its better to create a great design in CAD then realise it then do it in bits and pieces... the puzzle will resolve in about 20 design cycles I find. Keep it really simple, work on it being simple. Get that Z axis nailed before you do anything else... and a finance plan for the bought router as a benchmark...and a written objective for the machine. Always helpful to make decisions by... Peter

frankie 1 is a majority plywood machine cuts aluminium easily...