I’m a current student at St Catz which could (subjectively) offer you all of your requirements.
Catz has a large amount of ensuites where everyone gets one in 2nd year and some get them in 1/3rd year too upon request.
One thing you will find in Oxford is that most colleges do not offer kitchens and if they do there will be on kitchen for the whole college or one between many people. At Catz for all three years kitchens are shared between a max of 8 people and whilst small have everything that is needed.
Catz offers accomodation for all 3 years too,
As the youngest undergraduate college, Catz is one of the least traditional colleges so definitely one of the least posh. We have formal dinners every night if you want to go to them, but none of them require a gown and I’ve gone to them in joggers and there’s no issue at all.
The last thing is its location. Whilst it is not ‘central’ as in it is not on the main high street, it is only a 10 minute walk from everything you need in Oxford and close to all science and many humanity departments, and is on the same road as Law & English and Social Science libraries and departments.
Therefore whilst it is not ‘central’, it is not far from the centre and will tick all of your other boxes which most colleges cannot do.
As a younger college Catz was built to be more practical, with ensuites and kitchens therefore being more widely available. As such, with most Oxford colleges built hundreds of years ago, facilities are, often, much more limited and they often retreat to traditional and the old Oxford aesthetic so can appear more posh.
Other than Catz, have a look at St Anne’s, but that is by no means Central.
Most central colleges also do not offer onsite accomodation for all 3 years, so Jesus college’s accomodation for 2nd year is the other side of the city so that might be a factor worth considering
Any questions feel free to let me know or PM me!