Steel frame cnc router design/build

[JAZZCNC]

There is one inner brace already - 15mm plate & then 20mm outer on the end.

I'll look at doubling up that inner brace later.

Yep but the more towards the centre you can support it the better, bring that inner triangle inwards the gantry side does the same job so it's not needed there. On mine it's only built like that for ease of fitting it's piece on it's own that bolts on could have easily done it like suggested you do and would work just the same.

I have to keep an eye on the cost as its already adding up to way more than I originally intended...& its going to take me months & months to sort out the finances for it - so yes, the budget is important. I appreciate that its not worth cutting some corners - but I can't shove a bottomless pit of funds at it.

Yep completely agree and understand but just understand it's here at this point when building frame you can't afford to cut corners. If you have a weak structure you have a Weak machine and it will always be weak or hard to correct without major hassle and expense at later date.
I've seen this often where folks think I'll do it on cheap then upgrade later only to find out it's not that simple or easy and end up scrapping and starting again. . . . It's called learning the hard way, I'm just trying to help steer around that.!