I have about 125 Monarchs in process, from eggs to chrysalises. I've had lots of activity this year in eastern Mass. and still finding eggs daily. However, females will probably stop laying in the next week or so, if this year is similar to previous years. It is starting to get cool at night, and I think that might trigger them to start heading south or closer to the coast.
I've collected most of the eggs in my yard, but have also collected some at the milkweed meadow where I collect extra food for the large instars. The milkweed there is still in beautiful shape, especially considering it is a wild meadow.
As for aphids and other pests, I regularly patrol the milkweed in my yard from the beginning of the season and eliminate ALL other insects except for Monarch eggs and larvae. This inlcudes slugs, earwigs, spiders, beetles, etc. The aphids are pretty easy to control if you simply rub them off from the growth tips as soon as you see a few, and they never get a chance to colonize that way.
Also, I will often clip the tops off of milkweed plants that have degraded - this sometimes stimulates nice new growth tips, and the Monarch females love those. I agree that they seem to prefer the small plants. I followed a female around in the wild meadow for about a half hour collecting her eggs, and she was very choosy, and would find the teeniest plants hidden in the grass to oviposit.