Everyone starts with no upgrades, FGs are possible with them.
A lot of older players will say this. I disagree. Cards have been nerfed, the AI has been improved, and the easier gods don't pop up as often.
Personal opinion:
Continue to use your rush decks on AI3. Don't bother with AI5 or Top50 yet.
At the beginning, there's just no way to upgrade a card faster than 1.5-2 hours of gameplay. The reason I say stick with AI3 is that you can speed through matches, so you get more spins. This has several advantages:
1. If the first two slots match on a spin, you get +5 electrum. This doesn't sound like much, but with a speed deck you can play 45-60 matches/hour. At three spins a win, that's 135-180 spins, or up to 900 extra electrum an hour.
2. More spins means you can win more cards. If you don't need the cards, sell them. You may only get 15 electrum from the match, but most cards sell for 20-50 electrum. If you win 5-10 cards/hour (very conservative estimate with that many spins), it adds up quick.
3. Most importantly, outside of Top50 farms, AI3 is the best chance to get weapons. Use this grinding time to build up your weapon stash. NEVER sell a weapon (unless you get 12+ of one of them).
As you mindlessly grind through AI3, figure out what deck you want to use on the FGs. Focus on upgrading the key cards for that deck (most decks revolve around 5-10 cards). Once those cards are upgraded, start farming the gods. After you get good at that, you can upgrade the whole rest of the deck in the same amount of time it previously took you to upgrade about 3 cards.