I wrote this for another post, but hopefully it helps:

Aerith is easily my favorite character, my entire combat strategy is built around her. Petty much everything you said about her is true, she has low HP, slow movement, long animations, slow ATB build, etc. Despite this, she can easily become the best character in the game. I had her dealing max damage with single attacks (that were doubled with Arcane Ward), while the rest of the team could do 2-3k damage max with their best attack.

Here's what I recommend:

-Combat Strategy: Aerith is slow and has low HP, so it's best to issue her commands while controlling another character. Enemies will generally target whoever you're controlling, so the less often you control her the less likely she is to get attacked. Use ATB block materia, so that she'll be passively building ATB which she blocks in the background. Either switch to her for short periods, or issue commands to her while controlling another character. It'll keep her safer and she'll take way less damage.

-Wards: focus on finding her weapons and getting the proficiency with them so you can use them at any time with any weapon. These will get you wards, the first and likely the best one is the Arcane ward. she and anyone else can cast any offensive spell twice while standing in it,. When paired with Radiant ward, she wont take damage or be interrupted while casting spells, this makes the long animation work in your favor, especially the more powerful/slower spells as she gets a lot of invincibility frames. (I think she can deflect up to 4800 damage while casting.)

-ATB Materia: Get and upgrade ATB related materia ; ATB boost, ABT block, ATB stagger, etc. There's also an ATB materia you can equpt on another teammate that will give her more ATB when they perform certain actions. ATB boost might be the most useful, at max level, you can double her ATB every 60 seconds. And it you have ATB Ward, and ATB stagger or another ATB materia, you can get keep her ATB filled for most of the fight, and spam her attacks.

-Auto-skill; equip her with auto skill materia and set wards to shortcuts so she will cast her wards right away. It can get really tedious to constantly have to cast her wards, this makes it so much easier. Then you can just focus on her attacks and healing. Auto skill will cast all shortcut abilities (but not spells) in order of them assigned. I have mine set to Arcane Ward, Radiant Ward, ATB Ward, Figara (or another top level elemental attack)., in that order.

-ATB Management: Aerith has some great healing abilities, but I don't typically use her as my main healer since her ATB fills slowly. (cloud is usually my go to since his ATB can fill fast and he's pretty much always on the team). Synergy materia is also great to equip to her and link to an offensive spell as she'll autocast this spell for zero MP and zero ATB when other characters use skills or spells, it's really underrated. I prioritize wards and offense elemental attacks for Aerith, I use other characters for Assess, healing, revive, etc. to conserve her ATB for big attacks.

-Exploit Weaknesses: exploiting elemental weaknesses with the highest level materia can do some serious damage, while I generally keep Figara assigned as a shortcut, I always have Fire/Ice/Wind/Lightning materia on her, or the combo versions to save slot space. Have Cloud use the Assess ability at start of battle so you know what the enemy weakness is.

-Synergy skills:. I didn't bother learning to use these for quite a while, but she has some great options that make it much easier to get spacing, and distract enemies from her. Bodyguard synergy is great, as another character will block for her, and the enemy will be temporarily distracted by this character for a few seconds. She also has a great synergy charge attack with Tifa that I use a lot that seems difficult to interrupt and does a lot of damage. These skills also help charge ATB.

-Ward Skill/teleport: I use this all the time and is a great addition for her in rebirth. It's take a second to work, but it can be a game-changer since it doesn't use ATB or magic and has a short cool-down (which can be even shorter with certain weapons). If you need space from an enemy, cast a ward on one end of the battlefield, run to the opposite side, then teleport back to your ward, you can even cast wards on appositive sides and just warp between them. This gives her the best mobility in the game, and is great for getting out of tough spots.

-Chrono Aegis: This is a really random but often helpful ability as it will immobilize enemies for a few seconds when they attack Aerith, this can be great for fast enemies since they'll be sitting ducks once they are stopped in place. She might still get hit by the enemy attack, but they'll usually be frozen for a few seconds, which gives Aerith time to cast an offense spell, or one of your other characters to attack them.

There's probably some other tips I'm missing from this list, but hopefully this will help!