I'm pregnant with my third, and my oldest will be a couple of weeks off his sixth birthday on my due date. I will have a 3.5 year gap between my younger two. I am also 38 and will be 39 by due date.
In terms of mat leave and general logistics, I think a slightly bigger gap should be a bit easier at least while they are small.
My oldest is in Y1, younger brother is at preschool at the same school which is mainly covered by the 30 free hours, so I should get most of 9am-3pm most weekdays at home with the baby - more time than I had with my second because we could only afford nursery 2 days a week for a 2 year old.
By the time my mat leave finishes, the bigger two will both be at school, so we will only have to pay one lot of childcare fees. With my other two we had 18 months of 2 in childcare which was a financial squeeze.
Getting a new car to fit the five of us was slightly easier than for some families, as my oldest is in a high back booster car seat which doesn't need isofix.
My older two are both reliably toilet trained and sleep through the night (barring any regressions when the baby gets here). Having two in nappies, and then a toddler having wee accidents plus a small baby, was again quite tricky.
The older one was off school last week with a vomiting bug, and he was really sensible looking after himself and resting quietly, and managing to be sick in the toilet/a sick bowl. It was a different ballgame to looking after an ill toddler at the same time as a newborn (once I was sick into a bowl balanced on top of the baby I was breastfeeding, unable to leave the room with poorly 2 year old crying...)
Obviously it will mean a change in your family dynamic, OP, but only you can say whether that's likely to be a good change or a bad change. For me, I always wanted a full house, DH was on board, we can afford and have room for a third, and 40 was my cutoff point too, so I'm feeling very lucky that it seems to have happened for me. Other people would rather stick pins in their eyes than start again with a baby once their older DC are at school, and that's fine for them.