Buy a gas mig welder and learn welding, most home built routers are box section steel welded.
Since you are only cutting timber, you can keep wall sections to 4mm and use the cheaper round bar guides.
Your 4th axis should be incorporated into the table design, so you remove the table surface and the void below gives room to 4axis rotate your larger diameter work.
Probably have a moving gantry, 60 inches is quite a length so good luck with your grand idea.