This is a niche Synology used to cater to but seems to abandon more and more.

That's what Sandstorm.io was (open source, barely maintained now, acquired by Cloudflare)

Your sixth sense is accurate @TimTheTinker however it goes beyond just home for your awareness. This opportunity I see is below Oxide and targets any SMB that has data ownership and security for their business over time as an objective(+ many more). I am currently engaged in discussions with several niche businesses about building modular and extensible software specific to their industry needs of which the current industry software, written by non industry experts, doesn't support. Many business owners which contact me are disgusted in being fee'd to death so the opportunity to cut out those middlemen is massive and growing. Maybe this is your sixth sense tingle?

Yes, that absolutely jives with my sense. Targeting SMBs initially (since they'd have quantified cost savings involved), then expanding to home users in general after the product launches.

Ideally, an off-the-shelf machine (maybe from a few select hardware vendors/partners) with a very straightforward setup process, or an optional pre-installation.

I see data integrity as a key risk to address, though. Maybe something with a built-in Li-Ion battery (kept at 50% charge for longevity) that can keep running and safely shutdown in the event of a power blip, and an easy way to sync to rotating backups (i.e. plug in an external HDD once a week, or sync directly to another machine across the internet, no cloud in the middle). But IMO for most SMB-size work, RAIDs are overrated in the era of SSDs and modern filesystems.