I would challange the idea that Last Defender over Seraphim is the way to go for mythic+.
While the argument that Last Defender functions the more mobs you are tanking, it makes you tankier, holds true. It comes to a certain point where the scaling of the talent starts to yield less and less Damage and Damage Reduction per mob tanked. Unlike Seraphim, regardless of how many mobs you are tanking, you will always get the raw value from the talent given its a Flat Secondary Stat increase. Naturally this has the benefit of being of great value against a low number of mobs, in addition to being stronger on bosses.
From a defensive point of view. A Seraphim cast yeilds you roughly 10%/5% Damage/Damage Reduction on cast from versatility, in addition to increasing the strength of my SotR by roughly 7%. This alone is not something to shy away from.
When we then add in the remainder of the Secondary Stats that gets ramped up,which would be an increase of roughly 13% Critical Strike aswell as the boost of roughly 15% haste. Haste speeds up the recharge time of your SotR, aswell as letting you perform your rotation more rapidly, this naturally leads to more attacks that can trigger Grand Crusader, more Blessed Hammers and more Judgments. The Critical Strike has synergy with Avengers Shield from the Artifact Traits Bulwark of Order aswell as Tyr's Enforcer, increasing the not only the damage it will deal, but as a consequence, increasing the Shield the Avengers Shield cast will provide you aswell.
Keep in mind, that Versatility has the benefit of " Double Dipping " with Averngers Shield, the Versatility increases the damage the cast will actually do, resulting in a stronger Bulwark of Order, but what the Versatility does aswell as the previous, is to increase the raw Shield Value aswell, since the absorb shield counts as healing.
What i have been talking about for the last few paragraphs have been mainly from a defensive point of view, but clearly, all these Stat boots equate to a great ammount of your damage, a paladin that has Seraphim, and bursts propperly on packs, versus a paladin that doesnt have the tallent, is noticable in favour for Seraphim. This becomes Especially important in mythic+ where, in higher keys, you will be pressed on time, and the added DPS Seraphim will allow you to do, suddenly becomes very relevant, without even harming your abilities to tank that much.
Now with all this said, is not to discredit the usage of Last Defender, Last Defender has its place, and holds its value, and by all means, play with it if you enjoy and feel safe doing so. But i would advice, if you aspire to become a top notch mythic+ tank, is to become comfortable, and practise, an aggresive style of tanking, especially with Seraphim at hand.