So you're experienced and wealthy enough to have a good unupped deck against AI3 and t50. You have enough electrum to basically buy any unupped deck, plus you have some rares and a few upped cards. You want to know what the best mostly unupped FG grinder deck is.
Is that all correct? If it is, I'd spend some time in the FG deck building section of the forum. Find an unupped deck you like and an upped deck you like, and as you grind FGs and win upped cards, move towards the upped deck you like.
Spending some time in that part of the forum will give you more direction than I can, but here's a starting point: For unupped decks, take a look at SG's mostly unupped and variants. For upped decks, look at PuppyChow's 34-card deck, yaladilae's butterfly angel, and Fractal RoL Hope. FG grinding with a mostly unupped deck is going to be slow and painful. I'd guess you should expect a win ratio of 10% to start with, but the FG forum will have more info.
And good luck!