What to spend on?

I personally would save your orbs. I made the mistake of spending all of mine on trying to get Surtr only to be pity broken twice with not so great units(Leo and Innes).

Unless you get super lucky with the summoning, you won't get Surtr very high when you run out of orbs. You run close to hitting 2% at a minimum when summoning and spending 5 orbs in hopes of getting a single green isn't fun.

If you want to get a super merged unit for Arena, focus on a unit that isn't 5* exclusive, OR save up your orbs and wait a couple months to find a banner to spend them on a 5* exclusive. Also remember that 3.0 is coming out soon and there will more than likely be lots of demotes from 5* and from 4-5*.

Simple, if you aren't interested don't pull. If you don't like a character but pull for them anyways just because of hype, you WILL be disappointed after a while.

I have a couple rules I personally have. Ask yourself:

-would I pull for more merges if the character appeared on a banner in the future?
-Could I kill off a couple 5*s to give the unit an optimal build?
-Would I use this unit often?
-Does this unit have good fodder I could use?

if you can answer 'yes' on each (except the last one is good enough of a reason always) then go for it

I'll likely be saving mine for the Christmas banner. I'm glad to see Hrid, since I've been wanting him, and I don't have most of the other units on the banner ... but I'm expecting some good Christmas units, and I can always pull for Legendary units later. So I'll probably pull a few red/blue orbs, but I likely won't do too many on this Legendary banner. Basically enough to give myself the chance to be very lucky, but if I'm not, don't pour more orbs into it.

Any barracks can easily get by without Hrid. He's a good unit, but there are lots of units who can do similar things to what he does. You especially want to skip red because you already have the two other focus units.

Green I think you skip. S.Innes is not remarkable; he's a good unit but Cherche will do at least what he does if not better; he has some decent fodder but nothing enough to justify when he's sharing with Gunnthra and Kana and you already have Gunnthra.

Blue and colourless I think you could go either way. S.Cordelia is like S.Innes; they're not especially remarkable as units. I was thinking of some potential builds using her lance, but I haven't come up with anything or seen anything using it which is so good I find it worth prioritizing over other stuff. You already have B.Hector; more copies wouldn't be bad because you can fodder bold fighter. L.Lucina is definitely worth having, and if you get extra copies distant guard is good fodder.
S.Noire has a reasonable bow and infantry rush; I don't find infantry rush to be an especially compelling skill, and Nina's bow in my opinion is better than hers, but nothing here that most people should be in a hurry to get.

Fire and Ice is offering better drop rates for desirable 5* exclusives if you ask me. Definitely on green, and probably on blue also. Chill spd and spd tactic are really good.

But you may also want to just hold on to your orbs as long as you can to see if 3.0 gets revealed before Fire and Ice ends. Akariss believes it's possible there will be major changes to arena; we might get other major changes to the meta; there's also talk about obtaining old seasonals as 4*s based on a detail in the previous Feh channel. Generally speaking, it's a good time to be hoarding resources, avoiding spending. Major changes may be imminent, so any investment is higher risk than usual.