Hey, don't know if someone already wrote this but... Just a small guide for the Sans true ending because It took me like seven hours or more lol.
(SPOILERS AHEAD, kinda)
Ok so you need to do a normal route first, beating omega flowey. At the end you have to click in the sans portait, it will unlock it.
Now you reset and go to the date with sans. I don't know if it does something but be honest and say "pacifist" when he ask what do you remember. Okay, you did it and Sans said that he doesn't trust you... Well, reset and do his date again. Still? AGAIN (I think you have to do it 3 times)
So now Sans trusts you :D
Now you only have to do a pacifist route (it's your second so Asriel will appear as the final boss), do it like normal. You can go to the date with Sans again but if you want the ending you have to complete the game.
If you played before, you should know that sans lost soul don't rise the LP and FP, now it should.
When you complete the game, pick the Sans portait again (the lock won't be here now) and tadaaa you got the Sans True Pacifist ending FINALLY.
I almost give up before I finished the route... If this helps someone to complete this amazing route I'm happy 🥲 (Also, I'm sorry if my english isn't good, at least I hope it's understandable)