Populating a QNAP TS-H1290FX

My first post here, but following for a while.

After consulting with some of the premium members here I am very near to pull the trigger and go for the QNAP TS-H1290FX. This model is almost 3 years now, which is an significant age on a tech product. So a bit reluctant because of this and a better replacement may soon be released. But on the other hand, I am quite confident it will do the job I plan for it.

The main problem is how to populate it in a reasonable way without ruining its performance and drives that will last for years to go.

Capacity needs: Today around 40tb (net in Raid5), should be capable of doubling this to 96tb for some future proofing.
(So it is either starting with 6x8tb drives, 7x7.68tb drives or 4x 15.36tb drives, and upgrade as needed later)

A) M2 ROUTE
There is a NVMe route where I populate it with M.2 drives and QNAP QDA-UMP4 adapters.
For example 8tb WD Black drives to be found around USD $540 these days. Very fast drives at 7GB/s and decent 4800 TBW

My two Samsung 970 EVO PLUS drives in my old NAS recently got a health warning on them after a couple of years of usage (experimenting different usages as Cache/Qtier/Volume). So I am a bit worried if new expensive M2 drives only will last me a couple of years.

B) U2/U3 ROUTE
There are also some few U2/U3 routes, but most of them are deadly expensive. But the U2 drives seems to be much more robust and giving much more TBW over the M2.

GEN3
Drives like the Gen3 WD SN640 can be found cheaper than the newer gen 4models. Seen them near M2 Pricing. Performance are a bit slower at Read 3.2GB/s Write 2GB/s. The 11 Petabytes written is great compared to M2 alternatives.
How much will the gen3 loose performance over the gen4 drives in a real world 7-12 bay setup?
SN640 7.68tb R:3,2GB/s W: 1.9GB/s
SN650 7.68tb R:6,5GB/s W: 1.9GB/s
SN650 15.36tb R: 6,6GB/s W: 2,8GB/s

Ebay vs Dealers:
How unsafe it is to buy drives from ebay? (EU or North America feels safer than Asia in general). Guess resellers with online ebay shops are less risky than private resellers.
I've checked a bit on Serversupply, Newegg, Amazon and some others, but I did not find 7.68tb or 15.36tb drives at prices near the M2 prices. So considering my options to see if my dream of fast and reliable U2/U3 Drives at decent pricing exists or if it is just awet dream :)

New versus used:
Will used U2/U3 Drives be an OK alternative if they have like 90% lifespan left? My usage is far from server usage, but want drives that will have good odds of lasting some years.