I recently started playing and just got Nero, Nero bride, carmilla, emiya(archer)

Nero is good but a little bit selfish with her kit.
Bride is very good to give support to the team, and bot of them, help you a lot with arts teams
I have my Carmilla as my mian assassin, cuz i don't have Jack, and she's pretty good, does a decenta amount of damage and charges her NP fast to spam it, thaks to her skills.
And bout Emiya i don't know what to say, i do not have him but I've heard that he's not that bad. I would like to know about him a little more too. Hope it helped bro :D

1. Nero Bride is a pretty good single target Saber overall anyhow, and her skills make her also a decent support if you buff others. OG Nero has nearly no support capabilities and is AoE, but isn't bad.
2. Don't know I hear she's alright, but I don't care for her design to use her
3. He still hasn't reached his full potential with the strengthening quests they've been putting out for him. You can make one of his skills do a buff to all 3 card types instead of just Arts which will improve his NP, also his NP is good at star gen especially with the star gain up skill. The 3 arts card deck is a bit odd, but mostly left over from when he was being first designed, but it does mean he can NP pretty often. Overall he's alright and a nice farmer.
4. I guess as long as you're doing Nero Fest right now it'd be easiest to get the materials needed to invest in Nero and Nero Bride for now, as well as Nero Bride being really good overall

Invest for all of them, they have different purpose.

1. Nero Bride is a playmaker A.K.A flexible dps and supporting role. Also good at Boss killing, but for me i use her for Boss killing purpose (that NP spam though)
While OG Nero no need to much supporting, she can do much on her own as Last man standing (if your team falling down) or soloing bosses, also can be used for daily farm.
I have both Nero.

2. Beside Jack, Carmilla is a really strong female killer out there. Been with her for more than a year, she always deal great damage against them ( FGO servant mainly female ).

3. Don't underestimate Emiya, well he's a magikarp ( a fish turn into dragon thing )
Ultra high crit damage for later buff and he's extremely durable against saber. Can generate his own crit star, fast NP charge, high crit damage well.....he's a protagonist after all.
I have him too.

4. You have VERY STRONG servants there, raise them all.

1. They're pretty different and both are good in their own rights. 4* Nero Claudius is a tanky Saber with good NP spam, making her well rounded and independent with a good potential for solo'ing fights.

Nero Bride has a more supportive role with her kit but still has solid stats, especially a given since she's a 5* giving her a nice balance between support and still being able to dish out damage on her own.

2. Carmilla is a pretty good assassin that can generate stars and also crit herself, and with her NP it's ideal for boss battles especially against female bosses taking extra damage.

3. Emiya is about to get his 2nd strengthening soon this year which will give him Hawkeye, allowing him to generate stars and crit like crazy. His NP does hit like a wet noodle but it does ignore defense and is useful for clearing waves or challenges like the Exhibition Match against Siegfried in this Nerofest.

4. Depends if you're lacking a strong servant in a certain class, but I'd recommend leveling your 5* Nero Bride first since she'll be appreciated as friend support while will earn you friend points, and 5*s in general just have great stats that makes them worth the investment, and Nerofest is a perfect time to get all the XP and mats you need too to level them all asap.

Should have enough time too to level the others eventually.

IMO Emiya isn't nearly as bad as people make him out to be, even before strengthening. Plenty of servants have 3 arts cards and buster NP (David for instance), but somehow people complain about Emiya all the time.
His NP is OK. Sure, it's nowhere near Saber Alter NP damage, but if you really want to see a wet noodle, look at Elizabeth Lancer.