Question & Clarification on the KotFE Time Jump/Lockouts

Hi there! Returning player. I finished all of the RotHC before leaving in 2014. I'm back and working on Shadow of Revan. Before I start the KotFE content I know that there is a time jump, companions leave, and some content is forever locked out on that character.

What exactly is locked? If I've done my Class stories and have the stories done for RotHC and SoR am I good? Are side quests still there? Flashpoints/Ops?

I just want to ensure that I do EVERYTHING before hitting the "OK" button and moving forward. I'm trying to do everything I missed in order. I don't want to lock my main "completionist" character out of any more content than I already have by just not being here for five years.

So are there any achievements, quests, content that I need to complete before moving forward? I'd appreciate as much detail as possible. I've looked for lists but I've been trying to stay as spoiler free as possible but with SWTOR being such a small game finding niche guides (hell finding current theorycrafting/class guides is rough) is difficult.

Thanks folks!