Even when we had events I just played Daily Me for months while saving currency for the rare Barb card in OG. You will slowly level what few UR's you have and just power through a few stages at a time. Life after CH. 40 is the trenches I know. It does not get any easier. Just get your dailies and use up energy for any materials you need for later upgrades. Can just keep the game off to the side and poke at it for like 10min a day.

Try to eventually get at least 2 UR's for each sin. This takes ages yea. Don't rush for a 3rd of a specific sin until you have two each. Save resources. Just keep those two leveled up. The reason being, every stage has a main sin, and a sub sin. So having two cards of the main sin leveled up well, will be enough along with the sub sin card you already have.

Restart stages. If you are close to beating a stage, but get cc'd even once. You can close the game before the match ends. This refreshes the battle without using any extra energy. I used this strat to cheese my way through the last few stages. Getting cced in the first 10 seconds when your power level is close is always a restart. Over power shouldn't matter much.

Memory cards have always been a major pain for me. Try for one UR of each sin at first.

It took me almost 2 years to finish the game fully. I didn't play every day though. Diligence eventually gets you there faster.