I kept a turtle for about 10 years (outgrew everything, and ultimately was re-homed to a wildlife park, after he decided that I was his next lunch.
I used to feed him on reptile sticks, as a staple, and supplement with the tasty treats as needed - they need the calcium, ( I used to add coral sand to his tank as well), as try to ensure a balanced / interesting diet.
They need large (70 or 80%) water changes each week, and the substrate needs to be kept clean as they're messy little so-and-sos, and a nice area to bask - as you've already found.
It also does them good to have a circadian rythm set by lighting too - Tim had lighting for a regular 12 hours per day - he was always happy, curious and entertaining for the whole of the time I looked after him.
as his shell was healthy, it would happily wash with filter wool, and if he'd spent too long on the bask, I'd clean his shell with a little peroxide, and re-feed the shell with a little food safe oil after, making sure to not let any of these materials in contact with his skin.
Well kept, they are wonderful, characterful pets with a very long life