pokerstars most recent updates have made it more resource heavy. You can turn down the settings but then it looks old and clunky.
They are heavily advertising Chromebook here on TV at the moment. The ads are really misleading, going on about security, no data loss, long battery life etc. And completely overlooking the fact that you are extremely limited in what you can use them for. In my mind it's basically a tablet in a laptop case, but without the
App ecosystem of a tablet. Tablet versions of poker rooms are also very limited.
Buying a Chromebook is no different to one of those older cheap Windows CE laptops as far as functionality goes.
Grosvenor poker recently moved from the defunct MPN to the iPoker network, and in doing so no longer have a client that works on Mac. They say they are working on it, but it's not been forthcoming. The only option seems to be web based, and I believe they aren't supporting tournament play yet on that.
As much as I hate Windows, it really is the only way to be sure you can do everything you want to do when it comes to poker, and a lot of other things.
The poker clients generally have pretty low system requirements so you need something that copes well with Windows 10. I would suggest avoiding 4GB RAM models unless you're comfortable upgrading the RAM yourself. You don't need fancy graphics for poker. SSD is not essential, but will give a better experience with Windows. 1920 x 1080 screen is helpful to squeeze a few more windows on the screen and still be able to read them.
I use an i5, I can't comment on AMD or Pentium/Celeron since I've not used one.