All defense/attack buffs are better as sub, because you can survive a attack, which would kill you or max your dmgoutput in few turns, so you kill the boss, before he kills you. and RST Up helps against debuffs, which are annoying, not for general use but still useful in some cases.
Blind, dizz, para, all those CC debuffs are not realible, especially as sub summon skills, which need 10 turns to be activited.
I also missed both events.
I personally will farm phoenix and the dark weapon, if I still have time, then 1x apo.
Phoenix will be the best f2p-light eidolon until St. Nikolaos will come out, so still 6 months. Even the gasha one in the base form gives only 40%.