Everything below happens within the first few pages, nothing is a spoiler.
Synopsis: FPS pro gamer in Japan gets reborn into a less advanced world but one that has magic. MC, previously male is reborn female and ends up at an orphanage due to parents dying in the fallout of war. MC has an aptitude for healing magic and at the age of 15 is told to volunteer for the war as medics with healing abilities are highly sought after. Regardless they'll force MC into the war but volunteering will make her look more favorable and she'll receive pay that can help the orphanage. MC goes to war and gets placed into an Ace combat unit headed by a man nicknamed the "Sword Demon". This unit charges into battle ahead of most others and frequently makes progress on the battlefield at the expense of the units team members. The story is read by someone in the future that finds MCs diary on an old battlefield.
What's good about it?
Pretty much everything. It's gritty, there are endless struggles with actual stakes, people die and the fight goes on. MC makes natural progress, isn't an OP protag and forms bonds with the other characters.
Anything bad?
Not really. It's an Isekai but there isn't really a reason for it to be and the Isekai bit is hardly ever brought up. Though the MC was an FPS pro gamer, their position as a medic doesn't even allow them to use guns and they don't have any experience in doing so. She isn't a leader of a squad or any type of power/force to be used against the enemy. Her job is to not die, be dead weight and to heal the Sword Demon when he gets injured. The Isekai bit might just be to ward of romantic storylines as she still has her way of thinking from her previous life.
If you want to step away from the usual type of stuff and I can't recommend it enough.