Considering Hosting NAS/Server

Hello SelfHosted!

I’ve been a Linux enthusiast since ~2006, but I still have gaps in my knowledge and I would not consider myself a “fully-competent” Linux server admin at this point in time. I have to read a lot and ask a lot of questions to figure out things more knowledgeable users may do in their sleep. I’m gonna call myself “begintermediate”.

I’m working on simplifying my storage, backups, and general digital hygiene. I have multiple devices split across two locations and I end up having to use hard drives to periodically move files back over to my main desktop for sorting and archiving. If I want to access older files, I have to copy them from my main storage on the desktop to a hard drive, my NextCloud, or whatever device I want to access them on. I would like to avoid this drudgery by moving my file storage to a NAS (don’t really even need access outside the network, though it could be useful if I understood it enough to keep it secure). I also hope to simplify by backups in some way because currently all my devices just back up to a different pair of portable drives one of which I hand-carry offsite.

Requirements:

4TB+ storage to start
Expandability, I don’t know how storage needs will change over time, but 32TB seems like a fair upper end before wanting to update the whole system.
Would like to be able to run a few docker images for things like media server, open project, restyaboard, etc. I’m not sure if it makes sense to do this on the NAS or just get a simple NAS and do this stuff in a VM on my laptop or with a Rasberry Pie.
I don’t particularly want to spend more than $600 to get started, but wouldn’t mind having empty bays for later as I currently don’t have too much data.

Usage:

1-4 TB (someday up to 32TB) of files (docs, books, photos, videos, device backups, configs & code snippets, etc.)
Video, Photo, Music Access via Android Devices
Video and Photo access via a media portal (like plex or open media vault)
Would consider moving nextcloud here (currently on the public cloud) if uplink is fast enough.
Some sort of access via iDevice would be nice in case I want to give another some storage space.

Questions:

Does it make sense to mix my uses, i.e. media server, open project, etc. co-existing with file server for my docs and general files. Can I segregate portions for only local access?
I don’t have tons of time to maintain this. Nextcloud hasn’t been a pain, I log in here and there and make sure everything is updated (nextcloud and the server) and I run the NextCloud security scan to make sure I get an A+. Does it make sense to go for something like the better Synology NASs that can run docker images or would it provide better affordability/functionality to use a mini-pc or a FBmarketplace/craigslist slim pc hooked up to a drive enclosure or something else frankenstein-y. I don’t mind doing basic maintenance, but I can’t afford to spend every other weekend rebuilding things.
I have a dead WD MyBook Live and MyBook Cloud on my shelf. WD never updated them to fix the critical security issues, I missed the 40% off upgrade window, and they’re not safe to run with network access. They also sucked even when they were new. I want to avoid products doomed to become dead-end abandonware before I’m ready to upgrade. Are there NAS brands that are known to be better/worse with this? How does homemade NAS fare as far as hardware support and having to upgrade/rebuild when OS versions change.
Can I purchase/build a simple NAS that I use for storage and serve the files for my media server through a different device like my laptop? Is this better/worse than just streaming from the NAS itself or will I not notice in most cases?
It sounds like some of the pre-built machines can use drives of different sizes which would allow me to re-use the barely used drives inside of the WD devices. Do any of the self-build solutions allow for this.
I would LOVE some book/media/community recommendations for digital hygiene and how to handle store, backup, maintain the deluge of information in our modern lives.

All in all, I would appreciate any insight on a solution that gives a good balance between features & configuration, affordability. and maintenance time-investment. I figure a community of enthusiasts is a better place to learn than marketing copy.

Thank you for any help you can provide!