We are suffering from the same situation like your group .. Our summons are not totally full everytime we get into war .. But the thing for us is, we have enough time to save and collect Chakra Packs because in our server, my group is the only remaining active group left .. Week 1 and 3 of the month are Local Great Ninja War where we compete against no one and Week 2 and 4 of the month are Cross-Server Great Ninja War .. So during week 1 and 3, we are collecting our Chakras and let the summons starve and we only feed them every Wednesday of Cross-Server Great Ninja War week ..

Our first priority when it comes to feeding is to reach at least 80% (no need to be full at 81% to 100%) of our Gamabunta Summon, Bunta gives more attribute and power increase to the member of the group and Bunta have more skillset and have higher damage outputs compared to Serpent .. If we still have some Chakras left (usually 5k chakras for us), we feed Serpent at least at 40% just to have some power ..

And i know this is duhmb thing to say but i don't care, i'm doing this way before but got tired eventually .. I own 2 major accounts which are my Main and my 2nd Main, aside from that is I almost have 10 more alts (my members that quits the game left their alts to me), and i use 5 of them to just do 1 Daily Mission/Practice and claim the first froggy that gives Chakra Packs .. I also do Group Wheel at those alts for at least have a chance of getting Chakra Packs .. And when Cross-Server Great Ninja War week comes, i transfer all that 5 alt accounts to my main group just to sacrifice chakra and put them back to my other dead group .. In that way, we can keep up our summons feed up to 80% both but it's too tiring and eats a lot of time which i kinda have but too lazy to do xD ..

You may opt to do the same thing for now as we don't have any choice for us to choose to get more Chakra Packs and this is maybe prohibited by the game by they should not be hypocrite enough to know that doing this kind of way keeps the remaining players of older servers alive and active in this kind of events or feature of the game ..