DDR5 RAM - More Non-ECC, or Less ECC?

I'm considering building an 8x16TB ZFS raidz2 TrueNAS machine with a B650D4U and Ryzen 5 7600. I've got most pieces figured out, however I can't quite figure out the RAM.

I'm wanting to take full advantage of everything available such as dedup as well as running some services and game servers. Everything I read is that TrueNAS absolutely eats RAM, and the more the better.

First of all, I don't know how much to go for. I've heard 1GB/TB floated around a lot, but 128GB of Non-ECC is ~£300, and ECC jumps to almost £700. I don't know what the sweet spot of price and capacity is really, in terms of what's actually necessary on a server with this much capacity. Considering ECC ram is more expensive than the rest of the server minus hard drives, it feels like 128 ecc isn't really the right approach.

I can get a decent enough deal on 128GB of Non-ECC, but looking at ECC there aren't many good options. If I look at upgradability then the cheaper choices are 1x32 or 2x32, which are ~£170 and ~£340 respectively. 16GB sticks are actually more expensive, so it seems silly to do that. That at least means I could buy another couple of 32 sticks in a few years if I run into issues, but I'm worried that I'm underbuying ram wise to begin with.

Essentially I want to know, what is a better use of my money? Double the capacity, or ECC? Would 64 suffice for the size of array I'm wanting to run with dedup, or would I be desparate for more right off the bat?

One thing about the B650D4U is that it runs at 5200 MHz (1DPC) and 3600 MHz (2DPC). I'm used to the regular "dual channel is better" mindset, is that still true here? Is it better to run 2DPC at 3600MHz or split the two sticks between the two channels to run at 5200 MHz?

Any advice would be appreciated, thanks!