I found the argument in other places of the internet that Paladin can get his saves (as well as those of close by allies) into the sky simply by reaching level 6 and having high Charisma. Which, well, is true; if you would manage to reach Charisma 18 you have as much bonus to your saves as if you had proficiency in all of them, and double bonus on those saves you actually have proficiency in.
Yep, but you need level 7 for an extra boost to your aura. Ancients grants resistance to spell damage for everyone. Devotion is immunity to charm. Oathbreaker buffs damage to self and nearby friends and undead in melee equal to charisma modifier.
Vengeance only works on yourself, and gives you movement speed after opportunity attacks. Vengeance and devotion have just about the least level 7 benefits. Clustering around an oath of the ancients pal is pretty beneficial, especially since it has area heal anyway. It's very easy to give everyone bless and physical resistances alongside, all at cost of a bonus action. It's a very strong defensive class with some ability to deal single target damage. Still, you'd kill the gameplay by multiclassing ancients and losing level 7. 6 levels in pal is enough for vengeance multiclassing. Haste comes at level 9 and is vengeance exclusive.
It makes sense in a world where haste potions are banned, but if you're using them, it's a big loss compared to ancients' benefits. You'd lose both, say, sword flourishes and party face benefits from multiclassing, as well as Pal's true defensive potential (resistance and save distributor to party).
(Shar's upgraded spear of can make up for the innate lack of decent AoE options for Ancients, but needless to say the method of obtaining it may be a no go. It's an area smite. Vengeance and oathbreaker have that going for them, though it's a very hidden mechanic)