Welcome Back!
...as you've noticed, a lot has changed...
I'm a returning player who hasn't played the game since diamond tier characters were first introduced and new to the game.
So, that would mean you left between June - Nov 2018, since that was when there was only Evolved Diamonds and no Rift. Natural Diamonds and Rift Battles were both added in Dec 2018.
When I left the game my account was at level 50 with a few strong gold tier characters to help me get by, but now that I returned to the game, I can't really enjoy Rift Battles at all due to being matched up with players with similar player levels as my own, who by now have access to multiple high level diamond tier characters, making near impossible to defeat and end up losing the rift battle.
Oof...yeah, I get it, you're basically 6 years behind...and I can understand the frustration, especially with Rifts.
Ignoring Rifts, you're just starting where you left off and well, some things have improved (xp boost, energy refills, more fighters/characters), so it's not all bad.
I'd take those strong golds you got and make them Diamonds, concentrate on Prize/Event Fights. Save Theo so when you hit Levels 60/70 you can buy the Hyper Fighter bundle for only 5k (instead of the 10k). Open enough relics and you'll soon have a natural Diamond (maybe not all the resources to max it out just yet...and speaking of resources, worry about Guest Stars a lot later.)
Maybe I'm missing something and I am eligible to some of the rewards, but it's still rather frustrating not being able to be matched up with players with similar power levels to at least make battles a bit more fair rather than a one sided stomp.
Yeah, to get the bare minimum rift rewards (bronze 4) you have to enter 5 battles (that means spending 5 tickets on battles) and defeating enough nodes to earn a total (over the entire week) of 3000 points.
For any rewards higher, you need to be at a certain 'rift rating'. One bit of advice on approaching Rifts, I personally, start my 5 battles later in the week, like Friday/Saturday, since the more active players will have already scored higher rift ratings. Also, losing a battle here and there to lower your rating helps find 'easier' opponents and you might even get a streak going which gives you a slight attack/defense boost. oh and don't be afraid to reroll your pairings if they look to strong (look at their profile)...and only do the free 2hr reroll, don't use Theo!
Again, welcome back, Good luck and have fun!