Absolutely, you should join!
I joined Army Cadets at the start of Year 8 after a trial day in Year 7. 10 people from my school joined and only 2 of us kept with it a year later, there wasn't loads of people at my squadron anyway so we didn't do much compared to other squadrons around the UK. Some of my best memories from growing up were made with Army Cadets and there really isn't any pressure to join the Army after it and most people there probably won't. It's still a really good place to develop key skills and will still look good on sixth form and university applications as there's lots of opportunity to build teamwork and leadership skills which you wouldn't necessarily develop at other extra curricular activities.
I managed to convince my best friend to join when we were about 15 and she loved it as it was so different to other activities she tried, she'd also done Bronze DofE. Once we turned 18, she started volunteering there as a way of giving back to it and a way to help out as it's completely run by volunteers and our squadron needed them. I plan on volunteering when I'm in a proper job out of university so I can also give back as I believe it is such an important free resource for teenagers as they're growing up. There's really no shame in joining it a little later as you'll still have a couple years left before you age out at 18, and it's a great way to destress/escape during A-Levels!
In response to your questions though, I was never that fit growing up and took part in other sports outside of school. There isn't much of a focus on fitness but with the star system there is a fitness "module" at each level that you have to pass but even then they aren't too hard so I wouldn't worry too much. There's an Annual Camp usually in the summer in each county where you meet other cadets so is a great way to make friends and you would usually do some fieldcraft or an expedition which may require walking for a long distance whilst carrying a rifle and big rucksack so that shouldn't be too much harder than how you would've found DofE anyway.
Diversity in the cadets I think would more depend on your local community and the people joining. The instructors are always friendly and accommodating and would be unlikely to have any issues. In my community there's a lot of Portuguese and Polish people so that was reflected with who was in our squadron but we also had some poc - it just reflected our community. If you have huge concerns then you could see if anyone at your school is in it and ask them or contact your local ACF and bring it up with them as they'd probably be happy to help.
My local cadets wasn't that time consuming as it was Thursday nights 7-9pm and Sunday mornings 10am-12:30pm but it varies squadron to squadron - you should be able to find this out by contacting your local one or if they have a Facebook page or website then it might be on there. Most cadet forces I think have it so its split into two 2 hour blocks in the week and would usually be in the evening, I think we were rare to have it on a Sunday. There's occasional company weekends where you'd meet other people in your county's ACF and another way to develop more skills (and the Annual Camp). There's also maybe competitions against your other local cadet forces and some days out but that's down to what your local squadron plans. The only "homework" I can really think of was when I completed my Junior Cadet Instructor Course which involved making a lesson plan but even then it didn't take lots of time. If you were to struggle with drill (marching) then you can always practice that outside of cadets, and revising for First Aid but there's no assignments or homework outside of that.
Each star level is split into the different modules of Cadet and the Community, Drill and Turnout, Expeditions, Fieldcraft, First Aid, Navigation, Military Knowledge and Shooting found here:
https://armycadets.com/cadets/army-cadet-syllabus-2/). For more detail into this, I'd suggest you speak with your local ACF and ask them but they'll follow the syllabus and teach each aspect so you can pass it when you get assessed but nothing was ever too hard or strenuous. The Expeditions and Fieldcraft aspects are usually done during Annual Camps or Company weekends.
Cost wise, I mentioned it was free and it is but the only costs are your boots (at most £60 but you can usually get them second hand or your squadron might be able to provide you with some) and insurance (£20 I think but it could be higher now). The insurance is paid when you join and you don't have to pay it after that. Only other costs might be company weekends and annual camps to pay for food and accommodation but even then it's still really cheap as I think my annual camps were around £60 which is really good as it would be 1-2 weeks!
I hope this is helpful and as said at the start, you should definitely join as you won't regret it and there's no harm in trying it for a bit to see how it is!