Our current MEGA era continues the trends that the Scarlet & Violet era pioneered
negate the Summon or activation, and if you do, destroy that card
Terastodon, Woodfall Primus, and Primeval Titan are probably the best cards for the "big green creature" role, but Simic Sky Swallower and Broodmate Dragon can serve the role as well, although they require greater deck manipulation to be able to hard cast should you draw either of them before finding your Natural Order
Garchomp is best used as an energy accelerator
What to do next Once youve searched through your cards and found the valuable ones, you have several options: For the valuable ones: You can either hold onto the cards, perhaps get them graded, and see if they appreciate in value over time