Hi
@Mel_P Sadly i don’t think this is your system at fault, Photoshop is notoriously bad at focus stacking especially on the bigger raws like those from an a7r v. We have been pestering Adobe for decades about this and given it still hasn’t been improved i don’t hold out hope.
Helicon Focus is a dedicated software for this which works miles faster. This is arguably the best one in the market though it does come with a price tag though a reasonable one in comparison to adobe prices.
There are others for less and i believe in some cases free though i will leave it with you to research the pros and cons of those.
don’t get me wrong, you may see some improvements from upgrading, just probably not what you will be expecting.