Advices for Paladin subclass, based on specific RP [5e]

Hi, I'm joining a group mid-campaign. DM is adding 2 new players, but im a "surprise" as the second one. (we will be 5 players)
I will be level 4 when i join.

i did some basic guideline for my character, i need to choose my class, then I'll work my rp precisely

Anyway, I'm going to play a paladin (100%), my basic RP will consist of:
- a 40/50 y old Dragonborn who was full of joy in his early life (he left is clan because he felt it was too much closed mind).
- Ironically, with age and experience, he became embittered and too much serious. (trying some time to find his old mindset)
- he has some passion about Dragon.
- he LOVES history, he thinks knowledge is a weapon somehow, this led to him being a guardian of some secret archives.
- at the start of "my" campaign, i will be chasing the other new player who stole some of those archives i should've protect

What Paladin subclass could fill this RP? only XGtE / TCoE & MMoM