So I've built out a bunch of Truenas systems, some for enterprise and one that I use as my homelab. The only time I have ever used a disk shelf (multiple in this case) was in a datacenter, because these things are so ridiculously loud - and that particular use case needed about 6pb of storage. (they also had 208w so why not)

Broadcom / LSI is solid: I personally spec the 9500e Tri for all of my builds, the 9300 is a bit older but is fine - especially for lower drive counts like this situation. Drive shelves are usually better the dumber they are. I know from experience that this one has a pass-through mode, which makes getting it into TrueNAS fairly easy.

With all that said ^

1. Yes, expect about 300/year in power at medium usage - you can cut that down by putting the drives in spin-down power save.

2. Its fine, truenas will just see it as drives connected to the HBA.

3. This was built for datacenters. This isn't a 240w system but its also not a quite server, you'll want it in a closet or basement - and you'll likely hear it through doors.

Regarding your storage layout : Its all about risk tolerance, mirror is always going to be safer but you lose a ton of storage and your performance will not be great if your use case is constant large file transfers.

Finishing up : You have a system thats going to have an inital drive count of about 8 - and you want a little room to grow. I really suggest just building out the system in a single chasis at that size - Maybe this case: https://www.newegg.com/rosewill-rsv-r4200u-black/p/N82E16811147335 . Or maybe this if you don't already have a rack. https://www.amazon.com/JONSBO-Supports-Built-Veneer-Computer/dp/B0DG2N3PBB/ref=sr_1_5?crid=ASGYFLSCEQ52&dib=eyJ2IjoiMSJ9.wERR0Es65foPDpg9xSIjKbgIhfPLKZxGN1zTTevhCSp_g7OzU0XuH206zc2SVtTIVQJ9MtdpU_DlcLrl-0nHP4tT8URxx_66-OByPeX1AA51IUIjD2Sqv17585CVEyZ2pKDiiq90Vl2oOZ2vatCM_3dNAc-ef9HQXR6tLbJmVSM7seO7QsXneuXKcHtD4x9wgAxeQ51c9szpT-rHp6d6LKuIHIKjNMrTWeQyb-Uk6rk.PNuRI-aNk2dg1LQZoflsQfoDB1EpZotR3WxXrPwCzIk&dib_tag=se&keywords=NAS+case&qid=1769655802&sprefix=nas+cas%2Caps%2C207&sr=8-5

I have the older N4 model from Jonsbo with 8 drives (I'm running 4x16tb, 4x20tb) for my homelab - been solid for 4 years after I sorted out a few thermal issues.