Totally you and fiancés business what you choose to do.
But you need to consider all the ramifications of your decision and I agree completely looking after someone else's dc for a few weeks is in no way comparable to caring for your own PLUS don't underestimate the power of hormones!
I was a Sahm myself for a time and enjoyed it well enough but had some difficulties with it too which I really didn't expect considering I'd been a nanny as a job I'd done prior to having dd.
I've friends and family who have done a variety of parenting options inc a fair few sahd and even single dads in the mix.
There's a lot to consider:
1 finances - yes you're earning more now will it always be the case that you have more earning power? The reality is even just minimal maternity leave could impact that. You also don't yet know if minimal maternity leave is all you'll need. I planned to work until week before dd born and return after about 2 months off (sedentary non stressful office job at that time so no reason in theory why I couldn't. I was also a younger fit and healthy mum aside from endo and had no reason to expect complications. Instead I developed GD, had horrific pelvic pain issues from quite early on, then ended up on complete bed rest and hospitalised with pre-eclampsia. Then the birth was traumatic and dangerous for both of us - again completely unpredictably nothing to do with the pre-eclampsia and ended in an emcs with dd in scbu) also my then husband (army) was posted to our new location just after we had her so I couldn't go back to that job, new location much more remote and in an area of high unemployment.
Have you discussed how you'll manage finances on one income? Will he have free access to all income or will he work it on an allowance? Do you have similar attitudes to spending/saving?
2 - are you going to marry before having dc? You say you're engaged but do you have actual wedding plans? Because anyone planning on becoming a sahp would be very foolish to do so without protecting themselves in this way. Not just because of the possibility of separating but also in case the earning partner becomes incapacitated or dies - it does happen and can leave the sahp & the children in seriously dire circumstances
3 - have you discussed how long he will be sahd for? What he/you will do to ensure he can return to work smoothly - training to maintain skills/keep cv & refs up to date, childcare if he wants to go back to work when the dc are nursery/school/high school age - when he does want to. Have you considered what you'll do if he doesn't enjoy doing it and changes his mind?
4 have you discussed opinions on childcare and parenting? It's rare for 2 people to have exactly the same opinions on everything childcare related. Will you expect him to parent as you would if you were the one at home? Although to be honest I don't think you can really know this until you're doing it. How do you resolve conflict?
5 as pps have said have you considered that if you were to separate your fiancé would likely get residency as the primary carer you would get what most dads get of eow and maybe a day in the week. Plus you'd have to pay maintenance. You may argue that fiancé is saying NOW that he'd agree to 50/50 but during a split such promises tend to go out the window
6 while your mums reaction is quite strong it's unlikely to be the only time someone questions this decision. So are you prepared to cope with that?
Depending where you live some areas would still find it unusual and it could be difficult for your fiancé to access support/build a support network
It's becoming more common but it's still far from the norm.
7 if you're planning on bf how will that work? Not everyone can express. A relatively minor point but still worth considering.
Nobody really knows how they will feel, what will be important to them as a parent until they are a parent and you're not even pregnant yet.
It's overwhelming and life changing in a way you cannot comprehend until you're there.