Pi 5 will work if you only have low expectation, i.e. single drive, don't mind running the multiple drives over powered USB hub, don't mind gigabit LAN speed, not many resource-intensive services can be run concurrently. I used to use it.
Cyberattacks - Use Linux (For home user, ubuntu server is usable. Rocky's stable updates are too slow for my likings.) and patch it from time to time. Use best practice (i.e. proper user account control, don't expose any port to public IP unless strictly necessary). If you still scare about the files, have another backup machine running and do daily rsync to do mirror-ing to that machine.
File transfer - Setup samba yourself / with OpenMediaVault. You can access your files via SMB (natively available in PC, requires third party file explorer to support on phone)
Media player - Host jellyfin in the NAS. If you have android TV, install jellyfin app and call it a day, otherwise get Chromecast with Google TV to act as media player. The jellyfin app provides nice UI to browse & play the videos stored your NAS. Jellyfin also performs on-the-fly transcoding in case of unsupported codec.
Bonus - Run Ollama as your free ChatGPT, home assistant as your smart home endpoint, NextCloud as your Google Suite/Drive replacement, Pi-Hole as DNS-based ad-blocking solution, Frigate as your smart CCTV (video streaming & recording triggered via object recognition)...

I'm running one (Ubuntu server + services deployment done in Docker) and it has been working well for past few years.

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As for the build...

Entry build
i3 12100 RM489
MSI Pro H610M-E DDR4 RM399
Kingston Value DDR4-3200MHz 16GB x2 RAM RM350
WD Blue SN580 500GB RM239
WD Blue 6TB x2 RM1498
Silverstone VIVA 650W Gold PSU RM299
Casing RM500

Total: RM3774

AI build
Ryzen 5 8600G RM1049
ASRock A620M-HDV/M.2+ RM389 (or MSI B650M Gaming WiFi for 2.5Gbps LAN @ RM658)
Kingston Fury DDR5-5200MHz 16GB x2 RAM Kit RM539
Zotac RTX3060 12GB RM1369
WD Blue SN580 500GB RM239
WD Blue 6TB x2 RM1498
Silverstone VIVA 650W Gold PSU RM299
Casing RM500

Total: RM5882

Since the drive is SMR, so it is not feasible to do RAID-1. Just setup daily rsync from first to second drive will suffice.
AI build contains processor with NPU and NVIDIA GPU. They can accelerate AI workloads such as Ollama, Frigate, automated image tagging by object identification...

I would recommend proper NAS drives for main disks. I killed many budget HDDs on my NAS.

Also, Ryzen 7000 setup has terrible idle power. Is 8600G (not CPU only) that good?

If you want to go the Raspberry Pi route, I would suggest looking into the Radxa Penta SATA Hat for RPi5 (SATA Hat Overview).

This is available from Allnet Be careful that you select the RPi5 version since the PCIe cable interface is different.

You need to be careful of the SATA cable connection. Although they sell a 5-bay NAS enclosure, the interface on the NAS enclosure is SATA only whereas the Penta SATA Hat only has the SATA+Power connector. I don't know where you can source the specially built SATA cables - Image 5 and 6 shown on the Allnet website.

I have hacked together a RPi4 based solution using the previous version of the Sata Hat (without PCIe, so it goes through the 2 USB ports), with a different 4-bay NAS enclosure (which came with its own power supply). The SATA cable between the SATA Hat and the NAS enclosure was the biggest PITA to solve (I needed Male-Female SATA cables, which is next to impossible to purchase nowadays).

I'd also suggest not to stack 2.5" HDD on top of the SATA Hat as shown on the Radxa website. If you put it in an enclosure, you will need some way to efficiently remove the heat, otherwise it will keep crashing (my previous experience). Once I transitioned to a proper NAS enclosure, I could also use 5.25" HDD in place of measly 2.5" HDDs.

Another caveat of using HDD is that nowadays, most consumer HDDs are SMR, which will give you grief if you try to create a RAID array.

If you would like to use SSDs, there are other solutions (Argon One V3 case) (which I have not tried, and has potential SSD longetivity issues when used in a NAS). There are even RPi5 HATs with 2 M.2 NvME slots.