Seems like too many leaflets for elder. But it would be nice to have closer views of a specimen that hadn't been lopped, if you find another example later.

Not saying it is a Sambucus but there are images of S canadensis with 9 leaflets, so I don’t think that rules it out.

Do these images also show such elongate leaves, with essentially sessile leaflets and yellow midveins?

^^^ Yes, those pics were taken just yesterday. Thanks to our global meltdown...it was 81° F then, mosquitoes are out again, and some plants are now leafing out again...

For example, here is Sambucus canadensis elsewhere in the park (pic taken today), for comparison. So, I don't think the OP plant is Elderberry - which has long tails on its leaves and its young stems are less woody, as shown.

Could you take more pictures of the original plant? Btw the stems shown in the first two pictures look like last year’s to me, not young ones. The new growth is shooting from old wood. I’m not adamant about the id but the stems are very elder-like with their pith and lenticels.

nb OP’s image of S canadensis has nine virtually sessile leaflets.

Here's some more views of these plants:

I was asking about the images floral was referring to. Anyway I would think if your plants are elders they would have the usual stink when handled. Usual for Pacific red elder at least - if most other species have this as well that would be a good hint to check for.

Those all look like Sambucus to me.