Only real drive I can see being a safe bet would be 980 Pro, hopefully Sony just allows any Gen3 or 4 drive to work tho and just says if you use a Gen3 or slower Gen4 you won't get same experience as internal drive
No, that isn't how this is going to work. It's not about "enjoying the experience of the internal drive".
How fast the PS5 can get data from SSD storage into RAM is a known specification and games designers will be designing games to utilise this - take the new Ratchet & Clank game for example.
If Sony allowed "any old SSD" to be installed and the game was no longer playable, the complaints would be massive. It simply is not going to happen.
The only drives that will be allowed will be those, that when combined with the PS5's custom controller, offer the same of better performance that the built-in solution.
At the moment there are about 3-4 drives on the market that, on paper, will be compatible.
But these are still subject to testing - all it would take, for example, is for one of the drives when 95% full for the sustained transfer rate to drop below the current SSD solutions rates and they wont certify it.
If you're planning on buying a drive now - don't. It can only offer disappointment, no guarantee that it will be compatible and the price of SSDs only ever go in one direction, down.
If you're holding out that you'll be able to use anything but Gen4 and then only the fastest Gen4 drives, then unfortunately there will only be additional disappointment.