Added steel channel to X-axis c-beam

[routercnc]

Supported rail will be better than the wheels you have now and my previous machine ran on those for a few years. But there can be some maintenance such as sometimes the bearing coming loose in the housing plus there was some wear on my set that I noticed when I took them apart and was the likely cause of it getting worse for chatter.
So if you stay in this hobby you will get linear profile rails in the end.
As for the gantry sides - looks like they are the drop down style to raise the gantry. So they need to be quite thick. If they are aluminium then they should ideally be 20mm. But if they are steel they can be thinner. Steel is 3 times stiffer than aluminium, just in terms of material properties, but then when you go thinner you quickly loose out due to geometry (which is independent of material) so . . . go for 12.5 mm over 6.35 mm.
Better still if doing a really good upgrade raise the bed sides so the gantry doesn’t have to drop down and you will gain much more than trying to get stiffness back in the side plates.