I am also 40 and had my second baby in 2023!
Personally, if I was lightly bleeding every 2 weeks, I would get checked out with my GP. It might be perimenopause, but it might be something else going on.
I found that after pregnancy my periods had changed, they were closer together (not as close as yours though) and really light.
After that they got extremely heavy, still closer together, and I started to get hot flushes and other menopause symptoms (increased migraines, anxiety etc).
I went on HRT 9 months or so ago. It has helped a good bit, but I also think I have huge volatility in my own hormones, which has been hard to control with HRT.
Have you considered the contraceptive pill? If I went back again now, I'd probably try this. There are some almost bio-identical ones now.
My periods did actually start to change before I was pregnant, I just didn't recognise it at the time. I do think crashing from pregnancy hormones into volatile perimenopause hormones caused me quite a few issues. But that's just my personal theory.
Before I was put on HRT they did an ultrasound and checked various other blood tests. I have a strong family history of early menopause though, so this was always the most likely scenario.
I did also get a lot of "well toddlers are tiring at your age" rubbish, before some blood tests and a gynae confirmed it was perimenopause.