every man has own opinion about it tbh.

Dont even think about staying in low lvl, at least until you unlock all treasures. Otherwise, you are wasting scrolls (free draws).

How old is your server, do you have cross server sage already?

Do you have space time available? If yes, then is your server one of the oldest, in the middle or youngest?

Whats your personal ranking in the server, how much power difference is between you and top, lets say 5 players?

Can you get rank 1-3 during sage right now every time?

Holding back lvl was a good strategy at really old servers (eg. u start now on server 1/2 and without holding back lvl, you wont be competitive in at least sage).

Lets not kid ourselves, as a 93lv/70k bp i still get to fight 70~lvls with 20k power in matsurii from time to time, so argument about matsurii is invalid. Sage indeed has some brackets, I'd say once you get cross server sage, check how you fit there. If u can still get rank 1-3, then u can stop leveling and farm advanced refines, which you will need several thousands of to finish upgrading that power source. Remember that a big downside of holding back lvl is that u give up some power that can matter during 9tails (good gold/cp source)

I honestly dont remember when jinchu treasures unlock. You'd need to check some old event threads when they were implemented to see what lvl they start. Also it's worth checking out when ultimate training unlocks, so you have available all features (charms give massive powerspikes as well). You can safely lvl up up till 70 because both space time and kage treasure unlocks, then see how do you do in sage. You mentioned you havent used those items, good. You will have a nice power (and speed) spike in the near future, since refine event should come in 1 or 2 weeks.

First of all, this game is meant to provide fun. If you will enjoy holding back your lvl after u fullfil conditions above, then it will benefit you in a long run, because of massive gains in advanced refines. However you still wont have some content (passives, mysteries) unlocked, which may hold you back in using some teams, which may result in kind of frustration or something like that.

Always wait for rebates. Here's math for seal gain to seal spent ratio:

10 seals - 1 rebate - 0.1 ratio

50seals - 5+1=6 rebate - 0.12 ratio

100seals- 10+6=16 rebate - 0.16 ratio

200seals- 20+16=33 rebate- 0.18 ratio

300seals- 30+36=66 rebate - 0.22 ratio

As you can see, going for the highest available rebate in this particular event gives you over twice as many seal scrolls when compared to going for 10seal rebate during 30 events. In a long run this is the best what you can go for and you should do it, as long as you can stay competitive in pvp while saving. If you can't, because other players have rares which you simply cant beat with your current ninjas, thats the only scenario where you should consider going for lower rebate than 300 (still, only if those 100/200 seals for 100% will get you a ninja, that will swing the odds in your favor). I know, that getting 300 seals seems like eternity, but this isn't a game you play for a week. If you want to stay in the top, you need to make a long run investments, patience is rewarded a big time in this game and let's you stay competitive to some extent even with players, who spend hundreds of $ (if they don't follow mentioned rules).

Look at your sage enemies. Are you already fighting 70+? Once you all unlock kage treasure and some people start to get more serious ninjas, thats when you will need to decide whether to go for instant powerboost (kage treasure, its a 75% of getting a good ninja, since chojuro and bee dont do much on their own but all other ninjas do, also boost to a random main, not neccessarily water) or wait for 200 seals to get instant super rare from jinchuriki X treasure. If you still will be able to stay competitive without using seals, wait for 300. The loss between 200 and 300 seals is roughly 14 seals(66-36+16), but if getting jinchu super rare will assure you 1seal per sage, then its only month to make it up. Unless you can get 64 seals (at 264 in total you get 300 from rebates, thus last rebate from 300) during that month from various sources , its still worth using 200 over 300 scrolls, on a condition you will get that 1-3 rank to get sage reward.

Argument about han countering all present blitz, water meta and fire konoha teams is valid, thats true. You simply decrease their dmg by at least 30%(buffs), remove 1/3 of their hp(shields). you dont get much dmg though, thats why farming sage might be slow. To make use of roshi, you need to survive to use his mystery, which you wont vs lightning blitz.

Sage helps proccing roshis chase only once every 2 turns, also applies all stacks of poison at once. Yagura might be a better substitute, because you add crowd control (acupuncture) effect to the team. Thats personally matter of preference, but iI personally have easier time fighting vs sage teams than yagura teams. Roshi, yagura, sailor is ok lineup. If you lvl up to 80, you might enter a new bracket, and all your current efforts of slow lvl will be in vain.

Yagura is better than yugito, because yugito offers high combos and res, while yagura has high combos, acupuncture on chase and utility with mystery. Both as a standalone ninjas fulfill diferent roles, but in this certain lineup what you mostly need them for is combos to make roshi go off on a pretty regular basis. My suggestion is pick a lineup that you want and slowly piece fragments of those ninjas. When it comes to power and ninjas, theres no sense in going from 1 extreme to another, either full power free ninjas or full ninjas and neglecting power. However it is important, that you have a power to back up your lineup. That being said, you shouldnt worry about that when you choose 1 team and go for them. Problem starts, when you go batsht crazy for every single ninja, resulting in neither full ninjas nor power.