Looking for advice and opinions.
I have a long running Channels server on my ~10 year old Synology NAS. Unfortunately it is starting to crap out. I am going to replace it with a Unifi UNAS as soon as I can get my hands on one.
I have RPi 5 that I've used for a couple different things but its doing nothing right now. Can I use it as a server and the UNAS as storage? Or is it still recommended to USB boot the RPi? Or is there a better dedicated server solution out there?
If you're highly invested in Channels, you should invest in the hardware to run it.
You're better off purchasing a Beelink Intel n95 based mini pc from Amazon and installing Linux on it to run Channels DVR Server.
The other thing is to consider just getting another NAS that has compute, like you already have. I know it's attractive to add a Unifi NAS to your Unifi gear, but as you can see, you're having to look for a compute solution because of it since it's nothing more than network attached storage.
You Rock!! The Synology was great but I use it just for file storage and Channels. The want to get a UNAS is more about making storage easy on my wife.
I'll probably get a Beelink. No clue how to deal with Linux tho...
Neither did I earlier this year. Then I installed Proxmox on my 13th Gen Intel NUC and now I have Plex and Channels in LXC containers and a Debian VM running docker.
If you need help, this community is fantastic and always willing to help. 
Edit:
If that interests you, @bnhf has a guide on setting up Proxmox, Channels and Docker Proxmox-for-Channels: Step-by-Step for Virtualizing Everything
I run Channels DVR on a NUC running Windows 11. You don't 'need' to install Linux, it is just the general recommendation. I've had no issues running it on Windows. I am switching to running it in Docker on Unraid though (which is Linux based). I think it just makes more sense.
If you are comfortable with Windows, most Beelink mini PCs come with Windows 11 Pro installed. The install of channels on windows is pretty much a download, double click and wizard. Once installed, follow the migration instructions which are very clear and easy.
If all your wife needs is SMB shares, any NAS will do. Having watched a few videos about the UNAS, it's pretty limited and not particularly integrated into the Unify ecosystem. Possibly the event log would be integrated saving you a place to monitor.
Discrete purpose built devices is one of the many goals. I used to run a lot of services and apps through this NAS (NVR, Channels, Home Assistant/Homebridge, fIle storage, email server, and even Unifi network management at one point), but one failed RAM module shut it all down and created havoc....and of course it was while I was travelling for work. Wife was pissed.
I have SMB share setup in my synology now, but she would totally be unable to administer or deal with it if/when something happens to me. The UNAS seems simple enough for her to handle in that way.
Tossing between a Beelink/Linux and a new 2-bay synology for just Channels.
Can you wife handel the Linux side? If yes, then go for it.
If he goes with linux then neither one would have touch it 
LOL, his last box failed...