I think SN850 / Rocket Plus / 980 Pro market is really waiting on Sony to announce their first drive whitelist. Yes, it's also waiting on products to hit the market, but I would expect a lot of that stock to evaporate when a whitelist is announced.
PS5 expansion will be an expensive investment, but everybody who owns one will be looking to add 1-2TB to their 600~GB space. (Extremely likely that only 7GB/s+ drives will be whitelisted) Phison E18 availability is also an issue as I think only Sabrent has a shipping product so far...
*External SSD can be used of course, but only for PS4 games over the 10Gbps link, so reports so far is all you need is a Samsung T5 or other relatively slow drives to get basically twice as fast loading compared to an external hard drive.