If you already own it, stick to ext4, run only 2–3 lightweight packages, and accept the sluggish UI. It’s a decent backup-only or basic file server on DSM 7.1.1. If you don’t own one, do not buy – the performance/age ratio is poor, and official support has effectively ended.
Even though the DS412 can’t officially run it, let’s look at why so many users search for in the first place. These are the features driving the demand: ds412 dsm 7