It sounds like your plate if pretty full with full time working outside the home as well as the regular work inside home.
I've been fortunate enough to stay home and work from home (however my work from home has been within the last 5 yrs when daughter was self sufficient). Most work from home moms are required to have child care, etc. We own our company and I would not hire a work from home person unless I knew their work ethic, discipline, etc.
For us as far as children, I never had an urge to have another child. I felt complete with my 1 which was 18 yrs ago. To this day, I have no regrets.
We are able to financially take care of her well, provide for college, provide a solid stable home and have a great family team. We enjoy our family just like it is.. ALL 3 of us. My daughter has never remotely suggested the desire for a sibling either.
Not once would I have considered another just for the fact to have a sibling for her. To me, that is the wrong reason to have another child. The choice for another child should be based on the planning of mom and dad and if they BOTH want to add to the family.
You can not guarantee that siblings will get along and be close with each other.
You can set yourself up so that your only child will not be burdened long term if you need care, your retirement, etc. We have things set up so that our daughter will not be faced with any financial burden due to our health, retirement, etc. We also are able to fulfill our parental obligation to provide her college education to her so she begins her adult life debt free.
There are SO many factors that come into play. I have 2 relatives that pop babies out like Pez dispensers because they love the baby experience and I feel sorry for the older children that are no longer "babies". One of these women jus had #5 and one had #4. If they were financially set up and able to care for all of the children respectively it would be 1 thing but I already see resentments building with the older children.
I will add that the stigma of "only child" is just that... stigma. You will find MANY spoiled bratty children with siblings. My house is routinely full of teens to this day and there is not a dull moment around here. The attitudes and behaviors are what YOU make and model for your child... they are not based on being an only or not.
Bottom line it is your family, your money and your decision. Good luck whatever you opt to do.