I was beaten to the punch, but I’ll still share my thoughts here. I was introduced to SciADV with the S;G and S;G 0 anime right after the release of the 0 anime in 2018. After thoroughly enjoying those, here's what I did for the main and significant side entries:
- Chaos;Head NoAH
- Steins;Gate
- Steins;Gate My Darling's Embrace (Side entry)
- Steins;Gate Variant Space Octet
- Robotics;Notes Elite
- Steins;Gate Linear Bounded Phenogram (Side entry)
- Chaos;Child
- Steins;Gate 0 (VN -> Anime)
- Robotics;Notes DaSH
- Occultic;Nine* (LN -> Anime)**
- Anonymous;Code
* With O;N, release order is technically after S;G 0 and before R;ND, but lore-wise it absolutely makes no difference whether you read it before or after R;ND. I personally read it after R;ND because I already owned R;ND on Steam but hadn't purchased the LNs yet.
** Also on O;N, if you want the full experience, read the LNs then watch the anime. This one is the weirdest entry because the source material is the LNs, it originally wasn't even going to be part of SciADV but its own thing called SVN (Science Visual Novel), and the LNs were never even finished. The LNs are the most complete, but the anime provides an ending. The downside of the anime is it has notoriously fast pacing, but I still enjoyed it. The VN and the manga are both untranslated and are considered inferior to the LN and anime anyways. There was talk of the VN getting an overhaul some years ago, but that never happened and Chiyomaru seemingly chose to move away from working on O;N altogether.
Technically, Occultic;Nine is also part of the Science Adv universe, but even the creator said the connection is force.
^^^
Some last notes:
This list is a really good resource for finding all the various side entries. At this point I’ve now gotten through them all, but doing so might take a while.
NS2C has some additional side material and ebooks to download.
Chaos;Head/Chaos;Child Love Chu Chu haven’t been translated/patched yet, but they should be coming out pretty soon.
Also, yeah, R;ND is basically a side entry. A lot of the game is pretty fillery, but the true ending is still worth seeing.
TL;DR: See Blond’s post for release order or my first link for a full ordering.