Considering the battle thunder team requires a lot to be good.
Water/Fire give you free SRs, which can deal with the content.
If you see some old hands, they usually focus on water first, because almost all water SSRs are damn good. Debuff, dd, healing, buffing, no weakness. Thunder is somehow glas cannon... You will have trouble, when you face strong bosses.
Actually the chance up is a trap....the chance pulling a SSR is always 3%. You may have a better chance for thunder, if you get that 3%... anyway, better than nothing.
build a reliable team first, then grinding events for weapons, tickets and jewels, then you can pull more kamis and unlock their harems for free.