Hi OP! It's a tough decision to make. ❤
For me, I knew I wanted two children, so provided all was well, it was on the cards for us from the jump. I had DD 17 months ago at 39, and I'm due to turn 41 in Feb. Time wasn't on my side so we started trying as soon as DD turned one. I had a c-section and was advised to wait until then.
I'm sure you already know the risks/statistics at our age, so if I had to give you any advice it would be to make your decision relatively quickly. And, if you DO decide you want another child, to start trying as soon as you can.
I'm very newly pregnant now and though I'm praying this baby sticks, it's honestly not a great time for me professionally speaking. I'm in the thick of a PhD, and we could really use a lot more stability financially. But a second child was a priority for us, and I knew I didn't have oodles of time to get ducks in a row. It took us a little over a year with multiple losses to have DD, so I also felt I had to plan for an equally, if not rougher road this time.
It might help to have some basic fertility tests done so you have an idea where you stand? Also, to think about a plan for how you'd resume your career in the event you did have a second child. I don't think life has to come to a standstill, but I think it takes a lot of organisation, communication, and support to keep all the plates spinning.
Do you have a partner? If so, what do they think? Could you rely on them to fully pull their weight/do more than their share while you achieve your career goals post-baby? I'm fortunate in that respect. No way I could do any of what I've been doing re. work without my DP, who has done so much solo parenting through it all. I think that's a huge thing to consider, especially if you love your work, which it sounds like you do!
Good luck with your decision, OP!