Buster sorry this is long! If you make it all the way through I will offer a virtual bunch of flowers!
Buster from your post it sounds like you feel work will be helpful to you and it is not a financial necessity to you. Is that right?
I agree with Kristina that after school club and anther person picking your little one up will be too much and I know you are already re-thinking things.
I can only tell you my experience, with my birth child who is now 9, that she went to nursery two full days a week when she was 8 months and loved nursery but when it got to school she could only manage one day a week at after school club. She just wanted to be home, I think it was to see me and possibly primarily to veg in front of the TV! But it may also have been to not have to interact with other kids or other adults.
I think if financial concerns are very great parents do have to try and work it all out and it is hugely hard. Having said this, having a happy and cooperative child is very important. My dd is happy at after school club once a week but she has said she would not like to go more often. Your child and others may be different but I do feel you need to go slowly.
The fact your child has only been with you a short time, your partner is in the forces and your child came to you at four are all reasons for going very slowly, IMHO, slowly with any changes. And school is a massive change for kids.
If you need to feel more like you I would try and take as long as you can on adoption leave and once little one is at school use the time when they are at school for you-centered stuff, go for a swim, a jog, a nature walk etc, meet with friends for coffee, do a cookery course, or just learn new recepies, re-vamp old furniture, or write a book, join the PTA committee, whatever floats your boat.
If you are a partner at work, I would suggest that you can try and arrange flexible work, work from home (if an option) or part-time so when you do go back you can manage stuff well.
I totally agree your little one will benefit from a happier less-stressed mummy but if that happier mummy also means he will be away from you more I think you will find it doesn't balance out. EG he will possibly be more clingy when he does see you.
From other posts it sounds like there have been difficulties and I wonder if you think those will go down or lessen when he is at school and you are at work? if you do, then I am not sure (IMHO) it will be quite like that necessarily. I mean some things may get better but others may not.
He will need to adjust to school and you will need to adjust to work etc so allow yourself as much flexibility as you possibly can. If you can afford to go back later and work fewer hours I would do it, at least until all seems more calm at home.
I really wish you all the very best and I know that parenthood, and to some degree and motherhood in particular (I say this as my DH rarely seems to agonise over any parenting decisions!) is fraught with guilt and turmoil!
I have no desire to add to any bad feeling. I just think it is fair to say that more people picking up or caring for your DH, in terms of after school staff and your mum, is unlikely (in my limited knowledge of adoption) to make things harder in the long run.
Please also remember these years move fast, my DD is 9! I can so well remember when she was tiny, when she couldn't even say 'computer' let alone use one! This is in no way to induce any guilt in anyone, I think when you relax a bit more and find some extra outside interests for you (aside from child and work) you will find it easier.
Going from not being a parent to being one almost overnight must be hugely daunting. I built up quite a local support network so by the time my child was 4 I had many friends in a similar boat, and I am guessing for parents who adopt this is not the case! So do continue to build up some friends and support networks before DS is at school and enjoy all the bits you can. It goes fast.