OK, without reading anyone else's:
Your 30s are (can be) awesome, make the most of them when you can. In many ways I felt most complete and myself then - the best version of myself; physically, energetically, intellectually, sexually - but, I wasn't always aware of it at the time. Also more anxiety and just busyness in some ways. From that point of view taking a 'maybe this is as good as it gets' approach and really making that as good as it can be, day by day, month by month, is more than worthwhile. Then you'll know you didn't waste what you had, when you look back.
Oestrogen; make the most of it while you have it, be that for enjoyment or procreation.
Attractiveness; linked directly to the oestrogen (it's essentially the same thing). Likewise, make the most of it while you have it.
Physical ability; want to run some distance, climb something, explore somewhere physically demanding? Do it now. With the right training, you'll do it better and more easily now than later - and you never know when the time will come that you can't do it at all.
Mental acuity and memory; they fade from mid/late 30s, albeit you can cover for them later with greater knowledge and experience. Do recognise and enjoy their value while it lasts.
So yes, you are very likely to be less stressed (depending on circs) and more relaxed and self-assured in your 40s but, do make the most of all the hormones, brain and muscle function that allow you to be stressed and productive now, and not just in others' service but to the best effect possible, for you.