The advantage of customs is you can pull from several of the tags the origins have one of the tags + their own quests essentially whereas a custom character gets to choose 2 of the tags.
The Origin characters have 2 tags and these tags more often go together for solution sets than the others.
There is no advantage to the Customs in the tag department.
In fact Origin characters also get their Origin tag. Custom characters do not get access to it. Which is also quite often the solution to problems, open up new quests, and new opportunities that ordinary tags do not.
Frankly I think you're just over-exaggerating the issue. The tags are a collective pool EVERYONE chooses from, then Origin characters get their own unique ones. Yes, they each have their own stories and motives (GASP) and will continue along whether or not you recruit them.
They get their own stories prewritten while a custom origin is essentially you roleplaying what you want. The XP is pointless to worry about; it might make the game slightly easier but it's not going to make the difference between being level 20 and 60 at the end.
The only time you'd really care about this is if you min/max and somehow hunger for ever last possible shred of xp. The only "advantage" of Origin characters is that you get to see the story from a different perspective and with a singular ability to add some flavor to their playstyle. The goal is that if you decide to play as an Origin later it's a somewhat unique experience.
Your complaints are nitpicking. You don't get Time Warp? Oh, well. It fits Fane being some demigod-like race called the Eternals. It's still a single skill, and your racials will change anyway depending on the race you play so depending on what you do some may or may not suck. He starts with "essential" gear, the hell does that even mean? You can also choose to, you know, stop nitpicking and go earn your gear like literally every other character, Origin or not. What the heck could you possibly want? For Larian to give everyone immediate excess to whatever the hell you call essential gear? Yes you might need to talk to Fane to get the recipe mask (which is in no way essential, by the way, as undead just need to be in full gear to not be detected as undead) but afterward you don't need him.
I'm struggling here to understand the value of your argument. These issues I'd call at best nitpicking. A single character gets an ability you personally believe is strong. What's your solution to that? Give it to everyone? And he starts with "essential" gear? Don't even know what you think that is, but I hardly believe it's essential.
So yes, vent, get it out of your system, and then forget about this.
