Energy is a real problem when farming and you can forget about training pets on a character you are farming with. If you are leveling up it isn't so much an issue but if you are legendary there is no way to have sufficient energy to do much gardening at all, and again you can forget about training any pet. Any character that is under legendary tends to level up every time they beat Mithraya which fills their energy ball and makes gardening and pet training viable. Legendary players need to be able to fill up their energy balls too, and so I suggest making it so every time a player beats Mithraya their energy ball fills up upon so doing even when they are legendary.
Speaking as a legendary player with two gardens...
No. Plain no. This should not happen.
Firstly, once you have the Trial run down, and you got an even half-way decent group, you can finish the whole run in 30-40 minutes. People would abuse this fact far more brutally than anything else.
Secondly, Mithraya is the last mandatory boss in Celestia. Once you are at her, you should be Legendary - the dungeon's whole purpose seems be to make sure those that exit it are Legendary players.
Thirdly, gardens are an exercise in patience and limitation. While you CAN have a garden that contains 200 plants, you need serious amounts of energy to maintain them - which offsets the huge amount of rewards you harvest from those many plants.
Instead of complaining about not being able to run a huge garden, limit your garden instead to a size that you can manage without emptying your energy.