Something interesting I realised while looking at defensive spells for a Paladin2/Sorcerer18 build (leveling from 1) is Shield.
Im thinking of going Dexterity build because i fell like im wasting draconic resilience from sorcerer in full plate, this means that at the starting levels im going to be a bit squishier (but mirror image potentially lasts longer).

Although i love the idea of using my reaction for things like Sentinel and warcaster, I'm having issues justifying to myself not taking ASI, so i will use Shield for my reaction.

Basically when combat starts, i charge in and smack the enemy. Probably get hit in turn which allows me to see how strong they are (we try not to metagame) and its doubtful they kill me in 1 turn. If they hurt like hell, then on MY turn i move out of their threat area meaning they get an attack of opportunity, then go for Shield. Shield lasts until the start of your next turn so will cover me for when they next attacks as well. By using Shield on your turn you effectively get more mileage out of the spell than just using it for protection against the monster that goes just before you in the turn order.

I'm sure many people do this, but still interested to know if this is a viable use of reaction / spell etc?