So, which bit about NOT needing a metadata vDev did you not understand??? You do NOT need to buy a second 4TB NVMe drive, nor ECC RAM, nor a new MB nor CPU, RAM etc. and no one has suggested you do.
Clearly you have NOT been listening to the advice of people with more experience than you.
Plus in an earlier post you were suggesting spending money on a SATA SSD and a new larger NVME - money which would be wasted compared to buying another 16TB drive.
If you defer the purchase of the 3rd 16TB drive until later you may well have a LOT of difficulty turning a non-redundant stripe into a RAIDZ1 without moving the data elsewhere, so avoiding RAIDZ1 now on cost grounds will cost you a lot more time, effort and possibly money in the long run.
(I suppose you could create a RAIDZ1 now using the 2x16TB drives and a sparse file as the third drive - and then you immediately delete the sparse file, leading to a degraded pool which is essentially the same 2x 16TB stripe as you are proposing only with an ability to replace the missing sparse file with a 3rd physical drive later and resilvering to get the redundancy back. But this is definitely not any more recommended than the stripe you are considering and it does add some additional technicality to the solution.)
Please listen to the advice that I and others are providing you - using our valuable time at no cost to you.