Fogado (Only lategame bow, meaning you don't have to focus on Etie/Alcryst earlygame, and can go with other units through the midgame bottleneck)
I totally forgot about Fogado being a mid-to-late game bow user! I've tried using him in his special class and I've tried him as a Warrior (the whole Warrior+Magic+Radiant Bow combo), but he's never really shined for me. Don't get me wrong, he was incredibly useful! But Chloe has always shined brighter in that role for me. I love Fogado's character though. His design is amazing and his personality is hilarious.
Ivy (I've personally found that without Lyn Ivy struggles immensely in a few areas, and I prefer Lyn on a physical unit for bow support, also feels outclassed by Zelestia)
I always just give Ivy Speedtaker and call it a day, and she turns out perfectly for me. She usually just goes from recruitment with a Bond Ring and Speedtaker to whenever I get the DLC bracelets or Emblem Celica back and still performs top-tier. I've heard people swear by Lyn on Ivy, but the few times I've tried it, it's been incredibly underwhelming.
Celine (Earlygame carry, competent healer lategame. Dex is too low for Ignis's mediocre reward though)
That's where Marth comes in, as he patches that up and gives her 3 chances with just Break Defenses, 4 chances when Engaged IIRC, and 7 chances for it to proc during Lodestar Rush. This is especially important during Lodestar Rush, where Ignis turns into what could be a dicey situation into a boss kill or give you wiggle room pretty easily. (Timerra is the same, but with 8 chances for Sandstorm instead of 7.) But Marth is the earlygame and lategame solution. In the mid-game, I give her Byleth. (And I have a hard time taking that away.)
Hortensia ( Me issue - I generally play the game without vulneraries and status staves, with the exception of healing staves, which eliminates a lot of what makes hortensia good)
Fair, but that's a self-imposed challenge, it sounds like. While I don't usually use Fracture, movement staves like Rescue, Warp, and Rewarp are the ones that make Hortensia shine. Their rarity and extreme usefulness, when paired with someone who can extend the range and duration on her own, and has high movement, is what makes Hortensia incredibly valuable. (Fortify also counts here, since other staff-using fliers can't reach S-Rank in staves.)
I personally play the game without second seals for everyone but Jean and Anna, so that's how I'll be basing this list.
I highly recommend trying out Martial Master Clanne! Even without too much investment (usually only Break Defenses, which can be pretty easy to acquire with a little grinding and Micaiah, and House Leaders later on), he's almost always one of my top units. He naturally has the speed to double, the Defense bonus from the Emblem gives him just a tad more bulk, and the Strength bonus lets him hit harder. Add in Lunar Brace(+) and natural access to Weapon Sync, and I'm sure he'd be a monster for you as he is for me.
Zelestia (Magic Flier without any of Ivy's emblem dependancy)
My Zelestias have always been good but never great. I've tried her with Celica (my go-to), Chrom (good for mixed units but not the best), and Camilla (I still like her on Ivy better). I may try her with Roy one day, but overall, she's always just never held up. I like her class and I like her as a unit, but every time I use her, Celine is just naturally better at everything but movement. Any suggestions?
Framme (Earlygame staff/chain guard support)
Framme + Break Defenses (and Lunar Brace maybe?) = Surprising flexibility. Combat viability due to Lunar Brace and Break Defenses, S-Rank staff usage, and Emblem Lucina's Guard ability but now at 100% for everyone. (And with Emblem Lucina's weapons, the ability to take down fliers and dragons if you need an extra bow for the moment.)
Celine/Hortensia (Frail staff support)
If you're only using Celine for staff support, I feel like you may not be using her to her best potential.
To me, Celine has always worked due to her flexibility. If you give her Marth, she becomes a 2-range sword dogdetank that can heal if/when needed, and the Lodestar Rush gives her 7 attempts to proc Ignis. Marth also gives her a very nice boost in Strength (for Ignis), Speed (to dodgetank), and Skill (again for Ignis). (Granted, her Levin Sword will need to be forged so that it's lighter, but it still works out great.)
Emblem Chrom works well on her as well, as she gains an extra 10 Magic when Engaged, and the bonuses to Strength, Speed, and Skill from Marth's bracelet carry over here too. Giga Levin Sword may not be the best Engage Attack, but with her increased Magic stat it can do a fair bit of damage.
Meanwhile Byleth is practically made for her. With Byleth, Celine can now Rally, heal, attack, break axe-enemies while being protected, and even refresh four allies at once. When Engaged, Byleth allows her to attack from behind even the backline, keeping her safe from counters. Her own combat viability and healing abilities mean she can recharge her gauge pretty quickly away from direct combat, and then get back to rallying or refreshing allies even quicker. (Maybe adding Draconic Hex will help her soften opponents from a distance here too. She has access to Divine Pulse and naturally has high luck, so low accuracy isn't as much of a concern.)
So, with Byleth and/or Chrom, you've got a solid healer and dodgetank mage in the earlygame, who can become a refresher and backline attacker in the midgame, to a unit who can do all that and more, such as debuffing with Draconic Hex from the backline in the lategame. Celine is a unit who can evolve as the game goes on, with minimal investment IMO.
Hortensia is a ready-made Staffbot. Just invest one Master Seal and she's good to go. And she's only frail to physical attacks, which she shouldn't be near anyways. By the lategame, nearly all of my Hortensias for all of my playthroughs was immune to magic attacks. (Granted, she couldn't attack back very well, but her purpose in those situations was to aggro the magic.)
Jade/Bunet (Could've joined like 3 chapters ago and still would've been bad)
I WILL NOT STAND FOR THIS JADE SLANDER! JADE MAY BE WEIRD BUT SHE'S ALSO VERY COOL. THAT IS ALL. All jokes aside, I get it. Jade is one of my favorite units, but she's usually a unit I have to willingly invest into if I want to use. That usually means she's stuck with Ike or the House Leaders. Both of which are good for her and help cover her flaws and weird growths, but other units can use them better. (Madeline or even Jean for Emblem Ike + General, and Clanne, Merrin, Alcryst, or even Lapis for House Leaders.) Interestingly, I heard about a Wyvern Rider + Hector outfit for Jade that sounded incredibly interesting. It supposedly carried someone's Lunatic or Maddening run, and I'm somewhat tempted to try it. I can see that being the case, since Jade has weirdly high Res, and Impenetrable increases the Res by 50% when Engaged and during Enemy Phase. Making her an Enemy Phase tank could be a fun idea...hmm...
This wouldn't help your ranking of her, as she still joins far after Louis and even Jean, and this requires quite the investment, but it's an option.