Thanks for the fast reply. I get where you're coming from. Ideally, I wouldn't use them either, but I'm not a strong enough designer to make quality illustrations on my own yet. I tried FreePik, but I didn't have any luck finding any graphics that have the right look but that AD could also handle. I'm steering clear of the sites that primarily offer free graphics because they tend to be lower quality, and the graphics that I'm looking for are more advanced than the clipart-style content you tend to find on free sites. Also, I think the stock image industry may be run by organized crime or something. There seems to be a lot of inbreeding and cross-ownership between many of the image sites, so people could be paying wildly different prices for the same source files. And I really dislike how most of them use a ridiculous credits system for purchasing assets. It's a burdensome and entirely unnecessary step.
I've seen a post or two on the forum about a marketplace for Affinity designers, which sounds awesome, but they don't appear to have gained any traction yet. It would be cool if there were a function in Serif's ecosphere where designers could contribute to a marketplace from commissions from other users so they'll have revenue tied to the content they produce for the site. I'd be first in line for something like that. Now that I think about it, it's probably worth a separate post to see if any Affinity ninjas are available for consulting work. I just don't have a feel for how big Serif's user base is or whether there's enough interest to accommodate a service like that.
I think I understand the problems with managing files in AD/AP that were created in Illustrator/Photoshop, and given the proprietary characteristics of Adobe files, I don't know what Serif can do to improve compatibility that they haven't already done yet. I have a feeling that Shutterstock is my best bet, so I'll probably spend a little money to find out how good their files are. I'll report back here with some feedback. I probably should document more specifically here what I've found on the other sites, but I get the sense that most people either build their own graphics, or they have a source that works for them so they don't spend any time in the forum discussions on it. But the long and the short of it appears to be that many vector and raster files that were created with Adobe software have proprietary content that AD/AP can't apply to the image and/or the files weren't saved in a format that can be understood outside of Adobe's products. I'm learning the hard way that there are some functions that are easier to learn to do myself than to purchase from someone else, but I haven't been able to make the time to become a competent designer myself because it's such a small part of my job.
I'll post anything I learn here for anyone with a similar challenge, but hopefully there are a few other members that have cracked the code of reliable stock graphics.