It feels... really overpowered... I don't like making massively overpowered mechanics in my own fanon, honestly. However, if you are making the card, I will accept it.
Let's break it down (TL:DR version on top):
...after your turn is over put it face up in the Final zone but the Stride stays.
Stride stays for another turn, allowing players to infinitely Gride for four turns in a row. Stride itselfs gives the vanguard massive power already, Bushiroad making it only for offense is a decent idea at most. Now letting it be used defensively makes it really Overpowered.
(Tell me how often a Card is able to hit 26k without Legion or Stride now, I feel out of touch, by the way)
Take this card from the Final zone and ride it,Put 7 cards from the Drop zone to under your deck
You don't lose cards when Griding, even Stride makes you discard cards who grade totals up to a minimum of three (Which can be from 1~3 cards). Combine that with restoring seven cards back into your deck (which all are probably trigger, I may add) and Quad Drive, I call overpowered.
When this unit Grides choose 2 enemy Rear guards and give its power to it and retire one of them.
Uh... Does that not defeat the purpose of giving the two enemy rearguards power? You might as well say choose 1 enemy rearguard and it gains power equal to the base power of this card, and chose 1 enemy rearguard and retire it.
Even then it isn't much help to the enemy... If it increases shield power, on the other hand... :3
...your opponent has 2 or more cards in hand
Okay, possible Perfect Guard, but I think 2 or more cards is too much for Gride. Make it a minimum of 5 cards, that should give the enemy a fighting chance.
End of explanation
While I like the idea of using raw power to just crush the opponent's defend, do keep in mind that you wouldn't like that to happen to you either. Tell me, would you like defending yourself against a 40k base attack (excluding boost, which most likely brings it up to 50k) with four drive checks?