I'd hold off on getting rid of any 4* + units unless you have duplicates. If you've got more than 1, choose the unit with the worst typing and get some achievement points from it. But Gumi is steadily releasing 7* forms for outclassed units. As someone who's foddered Loch, Edea (accident, but still), Alyut and Lava, I feel qualified to tell you it's infuriating.
Buying more unit space is always a safe investment - I'm at like 290 and feel comfortable there, with about 160-170 always taken up by units I don't wanna fodder + whatever imps, crystals, frogs and evo units I've got on hand. It's more than enough, but I got sick of having to stop questing to deal with unit overflow every 5 quests. The first Frontier Hunter I did surprised me with the amount of gems it gave me, so I grimaced inwardly, took a shot and blew them all on unit/inventory space. Sucked at the time but I have no regrets about it anymore.
Inventory space is even worse. One of the few gems purchases I've made went entirely toward inventory space. Again, a very safe, albeit painful, investment.
With your Elza, you've got Rosetta, Shera, Rigness, Elza, something shitty, and friend unit, right? The first four are really nice, so keep them evolved and sphered and they'll probably get you through all the quests up to and possibly including the most recent one. There's really not a great reason to summon anymore - unless you wanna fill that fifth slot withsomething- until the next rateup.
And daaaaaaaaamn Ishad you made my day. <3 Lunaris. Summoned her on my first day playing and used her until I quit. She was outclassed by the time I returned.