ASK NC

Current config: Synology DS220+ with 2 Ironwolf Pro 16TB HDD. Have several TB in photos (RAW & Jpeg), a few TB of videos, etc.
Need to upgrade to a larger unit with 4/5/6 bay.
Watched nearly 30-40 hours of your videos on youtube and read several articles on your website.
Shortlisted: Synology DS1522+ (or DS923+), QNAP ts-664 (perhaps Asustor AS7606t).

Based on your review of DSM, I would have gone with ds1522+, but has the following concerns:
i) Ironwolf Pro 16TB HDD is not officially supported;
ii) no SSD storage pool
iii) lack of hardware video encoding - not major as I don't currently use Plex
iv) need an Ethernet switch with 4port LAG (or $$$ for a 10Gbe card)

I like QNAP's hardware spec, but no BTSFS support (and to a lesser extent, QTS).

What NAS would you recommend for (mostly) photography use?

Yes, Synology are moving away from media and focus on business use only.
Qnap would be a great alternative, but they do not seem to introduce same functions as Synology, like btrfs and SHR.
Asustor and Terramaster do offer BTRFS. Their software is not the best, but there is no other options for you. You will need to compromise somewhere.

With HDD lists, you can ignore compatibility with plus series models.
I would go for ds923+ and figure out something with transcoding - if that's something i actually need for local streaming and 1080p.