Chosen Solution
Is there any knowledge on if the Xbox Series S could potentially support larger drives as the 512 GB is quite small, and the expansion card is ridiculously over priced at £220
Seems possible, the method is the same used on Xbox Series X. The expansion slot on those consoles are simply CFExpress, you can buy a CFExpress to m.2 converter and install another SSD on it. The Xbox S/X has SSD hardware whitelisting implemented, so you have to buy the same model number Microsoft got for SSD>