Here's a place where modders are your friends. There's a mod for all items already out there that just dumps a bag in you camp with every item in the game. Not something I'd do at the start of the game, but as you play, if you realize you missed something, just mod it in.

For example, I just found out that I made a.choice at the end of Act 2 that skipped out on a strong weapon. It's tied to a specific companion's personal quest, you have to bring them, and you have to select a specific dialogue choice essentially asking for a reward. I'm playing as a pretty run of the mill good guy, so I figured "hey, no need to reward me, just doing good for the sake of it." That was like 40 hours ago. I'd like to play around with that weapon, so I'm gonna mod it in.

Otherwise, a lot of the strongest gear is available in shops or along major quests. There are, however, a lot of items hidden behind obscurity. And the game does lock out old areas after a while, but usually prompts you before doing so. It's also probable that some things are story locked; make certain choices, get certain results. I don't think you can get absolutely everything in one playthrough.

I'd say it's a great enough game that the FOMO can take a back seat, but that's just for me.