Of those I think Gelert will give you most bang for your buck. Plus you don’t have a LB2 Green which will help your war defense. I feel you pain pal, my yellows are my weakest color too. Chin up!!
Another vote for Gelert.
I love Xiaotu, but she is fine even at LB1, same for Loki.
Waiting for a lucky pull sounds great but for me, the last usable hero came in february, so can not recommend waiting
I think that a team with and LB2 Loki and an LB1 Gelert would be more generally formidable than a team with an LB2 Gelert and an LB1 Loki. Loki seems to have flexible ways of game-changing, if he survives, which he may be more able to do with those stat bumps as protected by some initial special armour.