I'm worried about putting too much stress on the mount. I intend to use it for planetary and lunar photography but hovering that close to the payload is a bit concerning to me.
My opinion: you do not want to use the Star Adventurer for planetary imaging with a 5".
Actually you do not want to use the Star Adventurer for a 5" MCT at all.
It is too large, too heavy and works at too high power to be enjoyable unless replace most of the SA accessories, which is IMHO not worth the effort. I even doubt you would be able to balance it with the stock CW (and as far as I remember it does not use standard CWs. Which is good because this way you can not overload it. Mind that often with star trackers the payload is referred to the wedge and therefore both payload, CWs and everything sitting atop it has to be figured in).
The same mostly applies to the C5: yes, it is smaller and lighter, but its center of weight is probably too high for the SA to handle it with decent results.
On the other hand the C5 would be a remarkably versatile telescope, I have myself a 5" f/10 (the much larger and heavier Alter M500) and use it for planetary imaging (will not win the prize for the best resolution ever, but give a look at the threads about imaging with 6" in the planetary imaging session. And it is definitely fun), deep sky imaging* (ca 800mm at f/6 and full focal at f/10 gives a lot of possibilities), and of course for visual.
But you need a bigger mount, I'd say at least an eq3-sized one.
If want to stick with the tracker in my opinion the largest CAT you could use with it is the VMC110, but these are tricky and if are unlucky have good chance for a true piece of junk, therefore I'd set for the 100/1300.
*of course with the guide.