While the 100mbps connection is your WAN to the outside world optimizing the LAN / WIFI for your internal needs seems to be more of a priority.
For something custom you could opt for a SFF PC or PI device and dangle adapters off them. You could use the PC though to build something within reason for ~$500 with an AP for WIFI.
Size of the space and walls comes into play with the WIFI signals bouncing off them or being able to go through them. Construction of said walls makes a difference as well if they're drywall it's easier, plaster a bit harder, concrete... you see where this is going.
If the home is pre-wired for different rooms it makes placement easier for smaller nodes to extend the signals.
Moving video / audio files vs streaming them are different needs. Streaming them usually doesn't pass 20mbps unless you're going full 4K w/ high audio then you might be in the 50-100mbps range.
Overbuilding to not have any issues with whatever traffic you throw at the network is better than putting together something more basic and then having issues / inconvenience.
Hosting a "NAS" if you're talking about an external drive w/ USB plugged into a router then you're looking at pegging the CPU on the router and slowing things down or having performance issues like reboots. If you're talking about an actual NAS with RJ45 ports and connecting with an Ethernet cable then that's not an issue.
I used a PC as a router though and combine several functions into the single case for convenience.
Router / Switch / AP / firewall / NAS / DVR (Plex) / OTA recording and other functions in a single PC case.
While my setup is over powered by many standards I don't have issues with processing video or getting the max bandwidth out of my 1gbps connection. Moving files across the network @ 1gbps+ speeds over WIFI and 400MB/s with Ethernet.
Power is rated at ~400W but, if you add up all of the pieces it's marginally more as they all tend to be powered on 24/7 anyway. 400W is a few lightbulbs worth of electricity use. Sure there are options for individual components to run at ultra efficient W's but it depends on how many of those devices you're concurrently running to get what you want out of them.
most routers don't have the storage capacity.
then it's probably better to build/buy a NAS and use it as a router, assuming the OS can be replaced, or you're using a plain PC.
Yes, splitting router and NAS/ fileserver uses to independent devices certainly makes sense, especially from a security- and maintenance point of view.
If you really 'only' get 100 MBit/s - and don't see that to improve significantly over the next 18-36 months, there would be little gain from using an x86_64 system for these requirements. A used r7800/ nbg6817/ rac2v1k (or even its older/ slower ipq8064 siblings) can do this easily (including SQM/ cake, up to ~190 MBit/s) and gives you two good 802.11ac radios as well.
USB is not a good choice for continuous usage (overheating, tiny antennas, low range/ performance), even less with AP mode in mind (usually between buggy and broken).
Given that you don't have huge performance demands I would split this into 2 devices, one "server" and one access point.
RockPro64 + dual port Intel NIC or x86 + dual port NIC running FreeBSD or OS of your choice for firewalling, NAS etc Storage would be USB-attached storage in case of the RockPro64 due hardware limitations. If you get a big enough x86 setup you can also for internal HDD(s).
As for access point I'd go for either IPQ401X-based or something like the Totolink A8000RU (11ac, Wave2, Mediatek based, ARM64).
Just ot be clear, I'm well aware that this isn't the enterprise solution++ but it'll be more than fine for home usage.
In a commercial setting, usually without trained hands on site, mechanical stability starts to matter. Namely a nice case, all NICs inside the case and not coming loose if anyone bumps it. x86_64 can usually meet this (second internal PCIe card, if there aren't enough onboard ones), the DFRobot carrier board for the RPi CM4 or the NanoPi r2s/ r2c/ r4s (r2s/ r2c are not fast enough for 1 GBit/s) can also do this (all in a nice 'professional' case) - one port in, one port out (hard to mess up in the field).
USB3 network cards 'dangling' from an SBC (e.g. RPi4) or the "RockPro64 + dual port Intel NIC" path require more faith in the on-site staff not to mess stuff up (by accident), which is why I wouldn't be too comfortable with those approaches in production (unless the router is out of reach and locked away from untrained staff). Obviously this is less of an issue in a private setting (with someone knowledgeable (you) on site all the time) or if the router can be kept out of reach.
EDIT: a well made case can obviously alleviate these shortcomings, fixing everything in place - but I usually don't expect anyone wanting to take this kind of hardware tinkering (3d printing, metal/ wood handicrafts) for granted in a professional setting.
Yes, I would recommend some kind of enclousre. Personally I have one made for low profile PCIe cards. =)
What I'm getting at is that you don't really need to overengineer your home setup...
