Is your 5 year old not in school? She would not be at home all day every day so wouldn't be bored and at age 5 I would have thought she would be at a developmental stage where she was able to play independently without constantly being entertained. IMO I think it's more of a disservice in the long run when we entertain our children constantly and don't give them the opportunity to experience boredom and just be provided with the materials and prompts to cure that boredom themselves. If we constantly look to do this for any child, then along comes age 10 and they don't know how to play. Just like with daycare children, it is not our job to play with them, but to teach them to play and by age 5 this should be a solid skill already acquired where little and infrequent guidance is needed. Don't misunderstand me for being someone who doesn't play with my daycare kids or my own children because I do, but if I was in your shoes and ready to call it a day with daycare and the only reason to carry on was for fear of upsetting my child, I'd think long and hard about who this really benefits. Her ACTUAL needs or our insecurities as parents constantly wanting to erase any glimmer of guilt that we are not good enough, especially when a newborn comes along. I had two within 19 months and my son, number 2, was a high maintenance, non sleeper, long feeder. Fortunately my daughter was very placid and easy going and was very happy playing independently. Obviously your daughter is older, but as long as she gets her mummy time then she will be fine.
Make a list of other things you could do with your 5 year old if for instance she is not in full time school. OEYC have a phenomenal amount of programming which would benefit her socially and also give you the opportunity to interact with other parents and get out and about with your newbie.
Library programs, community centre groups, etc etc.
You are considering taking only a select few on and the ages 6&4 are not going to provide your daughter with a playmate for long as they will be in school too. And then hiring an assistant....makes me think that you would be better off financially WITHOUT doing daycare and would free you up to work your schedule around your newbies sleep pattern and give you the opportunity to immerse yourself exclusively in your two children which in my opinion would be more beneficial for your 5 year old when complimented with other social activities like what I suggested.
Seems like a no brainer, if I was in your shoes my doors would close and stay closed.