Honestly, its really up to you. If you go to an old server (lower number) then you're going to have a hard time and you're going to have to level freeze. The problem that joining an older server has is that you are very far behind. All the timed events are going to be hard for you until you get to about 35k-45k power. I had leveled up a second account to help for GNW because my group had low participation. At level 55 and 17k power, every matsuri challenge battle went to about 2 minutes in queue and then was matched against a 70k-120k person, all the sage worlds were with level 70s and 80s that had 35k-65k power, and I would barely get any damage in 9 tails event (about 0.25%).

Thats why I suggest level freezing at level 70. Basically, continue doing the event to get the rewards but stop progressing in the main story. (You stop around Itachi's Entrustment) Still do all the timed events and the daily practice because you are missing out on a ton of resources that are good for you. It's technically not a true level freeze if you do that but I was level 70 at around 20k power, my alt is now level 78 with 50k power.

Joining newer servers gives you more rewards for being at the top of the leader boards but the server might just die out after a month or 2 and all your work will eventually be worthless until a server merger happens (which also has a bunch of special events).