Help me decide: 4Tb or 16TB?

Looking to put together my first NAS (TrueNAS). Current storage is 4TB (3x2TB SSDs) in Windows, with a 4TB and 2TB external drive. Using around 7TB total, with a view that I'll need more storage. But I'm not sure exactly how much or over what timeframe. I'm of the opinion that I'd find a use for whatever storage I have, but I don't have any plans that immediately require orders of magnitude more storage.

The SSDs are being repurposed elsewhere, but I've got 2x 4TB WD Red drives from several years ago. They seem to be in healthy condition, around 3600 power on hours, SMART data looks fine, etc. But to do anything with them, I'd have to buy more 4TB drives, which based on £/TB seems like a bad deal (approx £100 per drive, so £25/TB). My options:

• Buy a single 4TB drive. Use 3 drives in RAIDZ1 for 8TB of storage. Total cost is £100, but doesn't leave much room to grow. Basically ruling this option out
• Buy 2x 4TB drives. Use 4 drives in RAIDZ1 for 12TB of storage. Total cost is £200, and 5TB feels like plenty of space for the immediate future
• Buy 3x 4TB drives. Use 5 drives in RAIDZ2 for 12TB of storage. Total cost is £300, same 5TB of storage to grow into but RAIDZ2 gives me more redundancy

Something about buying 4TB disks and paying £25/TB doesn't quite sit right with me. If I went down the RAIDZ1 route, you could argue that since I already own 2 disks, I'm paying £200 to get 12TB and therefore I'm technically getting ~£16/TB. But on balance I think I'd prefer the security of a RAIDZ2. And £300 for 12TB is still £25/TB if we look at usable storage (or £15/TB for the raw 20TB).

My other option is to ignore my 2x 4TB disks and buy all new storage. I'm not looking to buy used, and the best deal I've found is on Western Digital's website. 40% off when buying 2x 16TB WD Red Plus, which comes out to around £15/TB. So option 4:

• Buy 4x 16TB drives. Use 4 drives in RAIDZ2 for 32TB of storage. Total cost is £1000.

Technically, the 16TB are worse value. I'm paying more than 3.3x as much and only getting 2.6x as much usable storage. But as soon as I outgrow those 4TB disks, I'll need to buy not only more drives but a HBA as well. And account for the noise, power draw and physical space so many disks would take up. Although I was looking at a Fractal Define 7 so space isn't really an issue

Future proofing is a bit of a futile task. It might take me 5 years before I outgrow 4TB disks. Or it might take me 5 months. On one hand I don't want to waste the disks I've already got. But I also don't mind spending a bit more now to get something I know will last. I was kind of hoping that by writing my thoughts down the answer would come to me, but so far it hasn't. At the end of the day, it's all subjective and there's no right answer. I guess I'm just looking to bounce ideas and hear other viewpoints