If the camera audio is compressed, you will need to decompress it. You could make little edits in the transitions from sounds to background, and duck the background. Samplitude is a good choice unless you want to go high end.

Okay, I'm making some progress on this, but lemme sound you guys out on a workflow. I might save myself some time and work by using Audition's match volume feature for the clips in a sequence.

But, if you've used this feature, sometimes it doesn't work right. For example, one clip in my sequence is just some ambient noise, to fill in a gap: well, Audition boosted that clip to match the dialogue volume. But generally, the problem is that the adjusted clips also have adjusted background noise, so even if the dialogue is the same volume, there's still some tweaking, noise reduction, and the like.

It looks like a good strategy would be to a) run the Match Volume, and then b) tweak each clip to a closer match, using levels, noise reduction, and the like. Does that seem like a good strategy?

I myself never use Audition for anything, but you can certainly try it. Basically, try everything you can think of in different orders but keep everything at the highest bit level. If you just want to work on the low end, you can certainly use multiband compression, but I would look into parallel compression. Remember, compression on a crap track can boost the noise, you may have to go through any gaps manually and lower the levels.
There's different tools for different types of material. If it is music, you can use a doubler, for example, and you can even use that on voice to thicken it.
Different EQ and multiband compressors have different sounds. The Gerzon shelves, while somewhat dated, might give a smooth EQ, depending on the material. They are part of the Waves Renaissance EQ. Samplitude has a good mutiband compressor. But there are zillions to choose from.

if you use adobe audition's studio reverb with bare minimum settings to only affect 200hz and below, it will make cheap mics sound like senn 416 because its adding microscopic reverb to only the very low freqs. do that after a basic parametric equalize('D' shape raise 30hz, 95hz 2.5db, raise 972hz 4.5db, raise 1306hz 4db, lower the 10k freqs). and finally, multiband compression will bring out freqs more evenly. that is always the last step.

I got a lot to learn, then. But I do have a question that might be answered quickly. Is there some way I could add some bass or low-end resonance to a fairly tinny audio track? Not room echo or ambience, but some "bottom?" (And if so, what tool in PP or Audition would be best for this?)

@Brian_siano the first thing I would do is find a take that you have both shotgun and camera audio and put them in the same audio editing program. Play them side by side and switch between the two. This will let you hear what the differences are so that you can try to match the camera audio to the shotgun audio or vice versa. Once you can identify the differences then you can find plugins to do things like reduce ambience /reverb, reduce noice, adjust the tonal balance, adjust the volume level. I would look at Izotope RX 6 audio https://www.izotope.com/en/products/repair-and-edit/rx/features.html and see which version you need to solve the problems you are trying to correct. They have a 10 day trial period, so maybe you could get all the audio repair done in the demo timeframe and not have to purchase anything. If not, by the end of the demo you would know whether it is a tool you will want to have access to indefinitely. The other recommendation is to not just work on headphones but play it in your room through speakers as well. And to you original question, Izotope has a number of tutorials here: https://www.izotope.com/en/learning/rx.html

I don't work for Izotope, but own and use their products, so I am confident that they offer some good tools for your application.