"I only have about a minute or two to get what I can (for a 30-40% wc) or else I'd be doing a 90-100% every week, which is a no-no in a cycled tank, right?"
This is not true. I do weekly 80% or more wc on all my tanks ranging from 5gal to 100gal. Never have a problem and have been doing this for 5years. You will have a mini cycle if you do a 90% wc and change your filter media at the same time. I did this once and it was not fun but it took 2 days to fix its self. If you have a mini cycle with a Bactria bloom like I did adding extra surface movement so your water have more o2 is the key to fixing the problem.
I say do a 80% or 50% just don't touch your filter when you do a large wc.