I always worked full time and it worked fine and has been better long term. You have to think about your work for the 20+ years after they've left home too not just here and now and the time when frankly they don't want you around so much and want their friends as teenagers.
We had nanny until they were at school and then she continued part time because she had her own baby and then her own second baby which came with her but that was luck that we had that. Then for a bit of time we had someone who did some cleaning in the house and then the school run and bringing the 3 children home until 6 or whenever we got in from work and that worked okay - cleaner come whatever.
Some people have a live in au pair at £65 a week for the afternoon and taking to school bit but we never had a spare room to put anyone in and I didn't want someone living in. Now I've older chidlren, part time help and am often working at home so it's really easy with the 8 year olds.
In your position I'd go back to your old job, not do the school option.
As for summer holidays sometimes a grandparent will have them for a week or two. Sometimes our have done 2 or even 3 weeks of a day camp like Barracudas and had huge fun. One had a school summercamp every summer for 2 years just for the day and sixthformers also helped with it. Then there comes an age when you're happy to leave them a bit on their own too or we have much older children now who can babysit as well