Nice. Looks great,

What size of the steel profile you decided on? If 80mm or bigger, you don't need the triangular plates. Believe me, will not move. Reason - the short profiles will hold it firm enough. If you insist, make them L shaped, so they will not bother you fitting longer than the table pieces.

And as for the size of profile, i am with Jonathan. Bigger the profile, the better. If resonance, its easier to drill a hole and fill it with something later.

I said it earlier some where but one of the best ways to fight resonance is avoiding perfect symmetry in the machine, meaning all pieces spaced same distance and mirrored exactly. Offsetting each piece even by 1cm here and there will have enormous impact. Same when avoiding mirroring