OP we’ve three, and I’d love 4 but won’t for a few reasons…similarly to you, I still could age wise, and in terms of age gap - all of mine are 6 and under (but my youngest is two).
oddly age is one of the things that stops me, currently my kids can still be at a similar stage/enjoying the same day trips, holidays, Sunday parks etc. but for me, it’s not just the gap it’s the span. Already my oldest one, is often the oldest at younger kids things (eg farms), it still works as she’s 6 but she’d be 9 by the time the next one is 1….and maybe it changes again when they can do things independently BUT for example, at the moment she’s asked to go to the cinema - it’s not a venue my youngest could manage. She loves theatre - again, we can only do the toddler type shows. Know these examples are of huge treat things…but they matter to kids.
health wise, my own health wasn’t good in my last pregnancy. Unrelated to age, I had a major hemmorage at birth. Baby was fine but I felt incredibly lucky to get through it with both of us alive…and illness, ability and risk do place a part in my thinking about another. It’s not a risk free choice.
Aware your speaking of the change from two to three - I adore my littlest, but it’s a very different level of chaos from two (hence we’d nearly consider four as hey no, it’s already total mayhem). One frustration for me is how everything is built for smaller families. Day tickets to events are extortionate as a family of five and I often have to say no (as it’s an extra 20-30£ for us). Hotel rooms will rarely allow you in even family two bed suites. The car is tricky to navigate size wise and “gear wise” (eg camping or mobile home holidaying which we love, is easy as a five, but packing the car is a nightmare, and mine always have less sporting and play equipment with us than others as…well, capacity!! Even of bike racks!).
As adults, we get far less couple time and solo parent time. I love to exercise and I’ll admit - it’s fallen by the curb - as the juggle of work and different school routines and childcare settings is…just time absorbing.
I wouldn’t change our set up, but I do think we were a bit naive going into it at the financial expense of a third (but admittedly this is posted when we’re still at peak childcare costs, in a challenging employment market).
The pluses: my oldest and youngest have the nicest relationship, it’s not parental but my older two v close in age and watching their kindness is heartwarming. My youngest is the most stubborn kid, full of engery, thinks he’s 5 and does everything he’s not meant to. Learnt to walk when we lived in a building site at 10 months and has kept that kind of vibe since. It’s a battle to keep him safe as possible but it also makes it great fun (as they’ll all try anything and love adventure). When they are not killing each other it can be great fun.