I have heard about isometric exercises helping reduce BP (there was a study posted on HN in the last 4 months or so), but I haven’t seen that they have to be hand exercises - are you sure of this?
https://www.adityabirlacapital.com/healthinsurance/active-to...
https://www.washingtonpost.com/national/health-science/this-...
On a side note, there have been studies about grip strength and lifespan, including on small children who have never strength trained at all. Higher grip strength had a fairly strong correlation with longer lifespan.