Who should I dump my orbs on?

Look at your team and then judge what kind of unit you need. Celica, julia, ike, ninian, delthea are all top tier units and you can't go wrong with any of them. If i were the one to choose, i'd summon on hero fest banner as a 5% summon rate is very good and will most likely net you something you'll actually use in the long run.

These banners for me are more about wants instead of needs, but yea everyone's different with teams. I want Ike and Julia from hero fest and then any remaining orbs would be focused toward Celica and Delthea from tempest trials.

On hero fest like the last one, i just pull every color every round because 10% chance to get a 5 star is too good for me to snipe colors. Unless I have a 5-10 hero that is on banner, i personally wouldnt snipe hero fest banner since it is better to get as many pulls as possible at that %.
Since they are all top heroes (most likely), whatever you get would be nice.

I'm not pulling anything until August 7th when first summons become free and the 3*/4* rates flip. Granted, that's partially because I got very lucky and already pulled S!Corrin.

After that... I'll probably pass on the other banners in favor of Hero Fest. The fact that it starts at 5% is awesome. Plus, I'd really like Genny for Wrathful Staff inheritance, Ninian for merging, and Julia because I love Genealogy of the Holy War.

Depends on what you need want? Hero Fest is always nice since 5% rates are really good. I personally will try for Ninian since I don't have a blue dancer. I might want Julia since mine is -atk but I want Delthea for sure so I might just try for Sonya. In any case, save up before you decide, we are going to get a ton of banners

Whoah! 5% for focus and regular 5 star? So 10% chance for a 5 star!?