It is a tricky problem, and I don't know the exact numbers. My knee jerk reaction is to point out that we have not needed to use the draft to maintain staffing, the way we did for some of our major earlier conflicts, but I think that misses the point. I think @peterb hit on it a bit, in that basic requirements for participating in a meaningful way in many professional fields are increasing. Hell, when I enlisted, there was cuthroat competition to get certain job slots, and the biggest issue that recruiters had was getting qualified applicants who passed the ASVAB with high enough scores and who did not have some sort of other disqualifying medical or personal issue. The days of being given a choice of jail or Army seem to be well behind us. Not a lot of folks need or want unskilled entry level labor, and the folks who do, frequently pay under the table for it - its where a lot of the jobs in seasonal harvesting and outdoor work that I did growing up went away too when folks switched away from using high schoolers. I can't tell you about Police departments nation wide, but some of the departments where I grew up seemed to have the ability to be pretty choosy in who they hired - I don't know, are the applicants going down, or are standards going up?
The volunteering in person question is a tricky one. I get pretty frustrated with the stupid online "activism," but I have also seen quite a few folks showing up in person at various protests and events, sometimes to the point where they are willing to end up extra spicey at the end of the day. I have no numbers and I can't tell you what ratio are willing to put their money where their mouth is, but it is some.
I could just be constructing myself a narrative so I don't write my country off, but I guess I am a bit of an optimist as well that even if I really don't agree with folks, there is a certain percentage who do want to make a difference (and maybe also looking at the nostalgic look back with a more critical eye). I am more concerned that they are going to decide to make a difference by outlawing beef because cows fart methane and mandate that everyone join together with the global consensus because nation states are bad or something like that.