As much as I loathe getting thrown into sub-30 content or missing certain tools I've grown accustomed to I still prefer the system we have here to a system where you retain your whole kit with lowered potencies.
In the games I've played with the latter system very little was done to actually keep synced players in line with players who are at level for the content, the chief concern is preventing overleveled players from clearing whole maps with no effort. Assuming equal players, a character who has their whole kit of abilities, passives, special equipment bonuses and what have you is always going to outperform a character with a limited kit even if their shared abilities are functionally the same.

If preserving a sense of balance between players at different stages of the game is your goal I really only see two options: limiting the kit available at certain levels to keep everyone on equal footing or essentially punishing players doing synced content for leveling up by gradually decreasing their performance to the point where a level 20 character with a 1-2 combo deals the same damage as a synced player who has to go through their whole rotation.