I’m not an Apple user/owner (technically not true, I own some Apple devices, but they haven’t been powered on in years) so I will leave the Apple specific questions to others.
As for the NAS questions, lemme have a go at trying to hit them as I just yammer on:
Your question about drive speeds belies a greater question… how will you hook up and connect to the NAS. NASes (NASen?) are not normally wireless devices, so they want to be wired into the network. For many networks, this means they will probably be connected to the switch ports in the router. Without doing something special, you’re going to have a maximum throughput into the NAS of 1Gbps. 1Gbps approximates a maximum file transfer speed of 100MBps. If all the devices are wired, then you might see these kinds of speeds. If you’re using wireless devices, you’re likely to get far less speed. This generally moots the point of disk speeds. A fast NAS will be far faster than your network. This is the reason the NAS specific drives are generally 4500rpms unless you spend much extra for the higher end pro/enterprise models.
Speaking of HDDs you should probably get one of those NAS specific drives. They have different firmware which makes them give up quicker on errors, because they assume the NAS has its own error recovery process (the RAID process.)
If you get a two bay unit, the ONLY RAID you should consider is RAID-1 (RAID-0 is known as scary RAID, it’s just a pool of disks, and if any one fails, the whole pool fails.) Many people will tell you RAID is not a backup, and they’re not wrong. You absolutely should NOT rely on RAID to save your only copy. There is a lot that can go wrong, you could have a structural problem (disk directory corruption, for example), you could have a bad write (some weird problem in the NAS) which gets spread across the entire volume, or you could have a disk fail in the volume. RAID can only protect you from that final thing… it can handle when one disc goes bad… and gives you a window of time to replace it and allow the volume to heal. The problem is that the disks were likely all bought and installed at the same time, and in the process of repairing the first failure, other disks can be stressed to the breaking point and then your volume is toast.
Everyone has their own risk tolerance for these things, but the extra drives of redundancy aren’t free. For example, if you have 6 disks, and you do RAID-6, you can survive two simultaneous disk failures, but you also gave up 2 full disks of capacity for the redundancy… you paid for 6 disks, and got 4 worth of space. In this sense, a larger number of drives means the cost of the redundancy is amortized across the larger pool size. 2 disks out of 12 represents a lower cost for the redundancy versus 2 out of 4, but on the other hand, a larger pool of disks means you’re more likely to have one fail at any given age of the system.
Since the major cost of the installation is the drives themselves and not the NAS hardware, I would consider buying larger, even if you can’t initially populate all slots with drives. My experience with NASes is mostly with Synology. In their product line, I would consider an 8 drive model which costs one disk’s price more than the 4 disk model:
DiskStation DS1819+ $931.00
DiskStation DS918+ $547.00
(also, just in case you don’t know about Synology, the last two digits are usually the year of the model, so those two above are 2019 and 2018 models. The first digits are the maximum number of disks, so one can have up 18 (with expansion) and one can have up to 9 (also with expansion). The expansion devices are pretty expensive, so I wouldn’t assume you’d ever acquire one, you might just buy a whole new NAS. )
Another NAS product I am interested in trying is the FreeNAS mini’s. They use a different technology, ZFS, which is more expensive to run, but some nerds really geek out about its capabilities. (It’s more expensive because it needs more RAM, and they strongly recommend parity RAM so that memory failures don’t end up corrupting your files.) The FreeNAS mini’s seem to be roughly twice the price of the Synology devices of equivalent size.
I’ve rambled for long enough that I forgot your original questions, so I will wrap up here for now.