Not a bad combo for Party Face and Social Skills Monkey

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Which to take as first - this depends on how much you want Heavy Armour.
Save Throw Proficiency - Bard gives you DEX and CHA, Paladin - WIS and CHA. As neither gives you CON save proficiency (for Concentration), a toss up

Two starting lvls of Paladin would also give you Defence Fighting Style, good for a tank.
I personally would start with two lvls of Paladin - more for the Defence than Smites - and then 6 of Bard. I'd push AC to 22-23 (HA, Shield, Defence, one or two +1AC trinkets) and wade into melee, letting the Spirit Guardians do the work. As the 2nd Attack would come only at lvl 11 I'd leave it out of consideration, the Smites being a Nice to Have feature

If you Fall, simply play as Oathbreaker.
As I have never started Act 3 nor passed beyond lvl 9 I have no experience and can only ask - how relevant would that 2nd Attack be? Could Bard 8/Paladin be better than 6/6, due to extra Feat? The lvl4 Spell?
Also - if I am reading things right - at 6/6 ESL would be 6+(0,5x6)=9. At 8+4 it'd be 10. The difference is having one or two lvl4 Spell Slots.