Anti beetle: according to a J C Raulston Arboretum guy (don't recall his name, unfortunately) four o'clocks (Mirabilis jalapa) are poisonous to Japanese beetles. So, I planted a boat-load of those from seed. Also, hand picking into soapy water... Because it pretty much can't be avoided. All in all, it worked wonderfully. I only saw minimum damage to the uppermost foliage of my cherry tree, none to the roses, and they were gone entirely by the time my dahlias began to bloom.
And, as far as having to plant a gazillion or so dahlia seeds to get any worthwhile, not so much? I mean yeah, I get a few I don't like every year, and a few more that are nice but nothing special. However, there's always something I at least save into the following year. That means that out of, say, ten to fifteen totally new plants each year, when I get one or two keepers, my 'success' rate is up to twenty percent. As opposed to the rate closer to a tenth of a percent from peonies, this is pretty excellent.
And you're right that that rate is heavily influenced by the selection of seeds. I've never grown from a packet before, only seeds collected from what I felt were the nicest of my own plants. I don't hand pollinate, but I do select one of the two parent plants, and if I want something not to have any input then I cut it back hard a few days before the preferred parent plant opens any blooms.
For example, the pics below. The first two are this year's successful plants. I like the form and fullness of the first, and the color contrast from central to outermost petals of the second. The second does have the curly petals of the parent as well. The third is one that I dislike, mostly for the color. I'm just not into solids (or highlighters). The fourth is one that simply looks like the parent plant, but less full.
