slice of life, feel good
Gundam
Gundam doesn't really fit those descriptors at all 😅 But... there is the Build Fighters series, which takes place in near-future real-world. It focuses on fans of Gundam model kits, and technology that scans the kits and lets them battle. It's not slice of life, but it's (mostly) kids and low-stakes conflict revolving around competition.
Opinions vary on which entries are the strongest or weakest, but IMO the first entry (Build Fighters) is fantastic, and really nails the whole fan enthusiasm feel. A lot of the references are going to be lost on people who aren't already fans of the franchise, but it executes on the shonen format well enough to stand on its own.
Edit: Honorable mention for slice of life goes to The Witch From Mercury, which made a lot of people very angry for hitting some of those notes early on.
I make it easy for people.
"Start at the beginning and watch in production order. A bunch of stuff happens between/during those early entries but it's all essentially retcon, save it for when you have the context. It'll be more impactful."
¯\_(ツ)_/¯ That's my advice, do what you want with it.