Hello,
Your work experience list so far I'll be honest is staggering. I applied with much less. It shows good commitment that you have already planned this so far ahead and still have sometime to go.
Secondly, don't let anything get in the way of getting top AS levels i.e aaaa and top predicted grades i.e. AAA. Back when I applied you could get away with Bs - but as you said, you want the best chances and that fundamentally starts with your academic grades.
If I am to be frank, I would say that you have enough work experience to sink a ship and good extra curricular points.
Live a little
