Honestly I froze my level on 1 character. Don't even bother with low level Matsuri cause odds are you'll be thrown in with people who are level 90+

I set a goal for myself. Don't hit 40 til 10k Power (was 2k away and said screw it). Currently level 41 (cause I still need to upgrade star level of main) and I'd like to hit 15k by 50. But I think the biggest advantage to freezing your level is the seal scrolls you will acquire. Cause you are not wanting to go spend them in Seal Treasure (or Sage/Pain treasures) and I have acquired roughly 75 Seal Scrolls with the intent of having enough to get 3rd Raikage, Omoi, Samui.

This also gives you a chance to avoid being plundered obviously which will let you acquire power a lot easier. I'd personally recommend level 40 to attempt to stay there as long as possible. Mostly so you can focus solely on your refines to at least get them all to level 6. Then work on your Battle Armor when that unlocks. Also by staying low level you'll acquire a lot of items you can't use such as Refines, Battle Armor materials you may obtain from Monthly, or even Cave Keys. I have over 300 cave keys I still can't use on that account (Thank god Myoboku Trial lol)

Don't rush yourself and set goals and take your time meeting those goals because you will have a huge boost in power when you can start using all your materials you'll gain a huge power boost. And if you wait for rebates you'll get even more out of it.

Not really cause you still gather a lot of Battle Armor materials through events. But the point is to focus on 1 thing at a time for example, meeting a goal of all level 6 refines then level up to gain access to Battle Armor and focusing on that to a certain battle armor rank you want.