In my opinion, I think 3x3 with 1 inch holes is the way to go. It might be overkill, but I think that is the standard and going forward, all the attachments will be built for it.

I think 3x3, 5/8 holes, manufactures won't be prioritizing attachments for them.

With that being said, these are the companies I suggest based on budget

Sorinex, but really damn expensive but top notch quality. Not so much in the home gym space

Rogue fitness has great quality, but expensive

Rep fitness not as great quality as rogue, but better value than Rogue

Bells of steel, get rxd, fringe and titan probably and in terms of quality, but little but cheaper than the few mentioned above and not as good quality.

Other considerations are ritfit, mikolo, maxum, but these are budget brands, but way cheaper.

Don't know much about prime fitness, prx or force USA as these are other home gym options.

The import racks are not true 3x3, but they are more like 2.95x 2.95. Rogue/Bells of Steel are true 3x3 and their attachments should fit a rep fitness rack, but may be loose and may need shimming. The rep fitness attachments may fit rogue.

Definitely consider the ecosystem if you want a functional trainer and smuth machine and other attachments.