You are absolutely correct! and those as well as several others are the reasons why I am such a "wet/dry fan".
IMHO they just do not "scale very well" to a 29G tank.
With respect to water changes I believe that I just had an idea although I have not seen it set forth in the literature.
Also this is based on my limited experience with an Eheim cannister filter.
Why "not get into the return line from the filter to the tank";
install a TEE with tubing which is typically curled up but which can be uncurled to extend to a WC bucket;
install a valve above the TEE in the tubing to the tank and
install a valve in the WC tubing.
These valves and tees are very inexpensive and with their "barbed connectors" are fairly easy to install*
I know that this sounds "crazy" but I believe that if I had a cannister filter or filters I believe that I would "try it".
I do not know how else to say this but:
I am concerned that I have not said the correct words in order that a full understanding of the hydraulics of a wet/dry filtration system has been accomplished and
I would feel bad if you spent the time and money attempting to retrofit your aquarium to a wet/dry and it "did not work".