The decrease of BP in response to deep inhalation and Valsalva is very short-lived. But since you are interested in that subject, maybe you have heard of Mayer waves? When BP is continuously monitored, an oscillation becomes apparent, that is uncoupled from respiration, and follows a frequency of 0.1 Hz. This is called the Mayer wave (named after a researcher who discovered this phenomenon more than a hundred years ago; I have a paper from 1919 in my library, if you are interested). Fast forward to today: It appears that cardiorespiratory synchronisation with this Mayer-type oscillation reduces blood pressure. That is, a breathing frequency of roughly 6 breaths per minute (BPM) will have that effect. I had been in touch with a group of researchers/clinicians from Austria and Germany who have developed an end-user device that enables users to get into the rhythm based on monitoring their heart rate.

I'm unsure under what name they have registered the device, but it should be on the market already. I have put their paper on my site for you to download if you are interested https://www.adiphea.com/wp-content/uploads/2024/08/Device_guided_slow_breathing_with_direct.8.pdf