Zeline and Alberich could be months away with the tiniest chance to summon them even with exorbitant spending. Ive learned to not set myself back from further progression waiting for that ‘perfect’ hero when you’ve got a good option already. Greg is a good 5*, similar to Lianna, although geared more towards Titans.
Bring him to 70, if you happen to get Alby/Zeline before you get your last ascension items, then switch priority. Greg would still be useful at 70 for his crit utility and elemental link on blue Titans. If you don’t get Alby/Zeline before you get your last needed mats, ascend Greg with confidence.
I didnt even mention Melandor because 5 tonics is the right number to be making 5* plans, saving your sturdy shields.
Best of luck!