I feel like this bares repeating again since OP is very concerned. So lets dispel some myths.
1. Sony never said anywhere that you need a significantly faster SSD to match the performance of PS5 internal SSD. What
Mark Cerny said was
The SSD you put into the expansion port has to be at least 5.5GB/s so games that push the internal SSD will also work flawlessly with the expansion SSD.
The expansion SSD needs a little extra speed to compensate for 2 priority levels vs 6 in internal SSD. What they are talking about here is the prioritization of IO requests. A game says I need data A,B,C, 1, 2, 3 in this order. The IO grabs those file in the specific order the game needs it, the high priority files gets put in the higher priority lane and the lower priority file go to the lower priority lane. You do not need 7GB/s to compensate for only have 2 true priority lanes, ti doesn't work that way. Just being slightly faster means the IO request gets fulfilled faster. 6 levels of prioritization just gives finer control of how IO requests are fulfilled. Like Mark Cerny said, if you shoot someone and they need to say a particular dialog, that audio file can be put in the higher priority level to be fulfilled first. That is all it is.
Seriously a 5.5GB/s SSD will work just as well as the internal SSD. A 5GB/s SSD will work just as well, maybe a couple percentage slower at fulfilling the IO requisition. A 4GB/s SSD will be maybe 15% slower. That is a difference of loading something in 2 seconds vs 1.5 seconds. You won't notice the difference unless you put it side by side. OP you can sleep easy.