I got the dev room ending after a single play through and I was missing a lot of the best gear and wasn't stat maxing because I was not using a guide. It's not that hard.
I don't know about you but I didn't use a guide either (beyond the one built into the PSX version of CT), I just have a habit of hoarding stat ups since I can never be too sure about whether I want to diversify or to min/max. It just saved me a lot of trouble when I went and trashed Lavos with only Marle and whatever levels and gear I had on hand after my first run (it's not hard to bump into most of the gear listed by Annubis anyways since they're contained in noted treasure troves).
Also, I could be mistaken but wasn't there two points where you could reach the dev room ending in CT? Am I thinking of CC? Or am I dreaming of Shashami?