Hello Geoff. Indeed, the SW GTI will constrain the size of your OTA to something small and less than ideal for planetary imaging. You may want to look on astromart or elsewhere and see if you can find a vintage Meade ETX 90 (provided you can find an OTA only without the fork mount). That may be a good pick for your GTI mount. The planets have extremely small angular sizes and require long focal length OTAs in order to get any decent surface resolution.
In the long run, you will want to switch to a beefier mount and pick a large SC but in the meantime the old and fairly trusty ETX should work. That said, in case you buy it with the entire Meade fork mount, see if it is operational and works that way for you (and secure it to whatever tripod you own). Otherwise, just take the OTA off the fork and fix a vixen style dovetail on it .
Another alternative is to use a small refractor but taking the reducer off and switching to the visual adapters on the back (either 1.25 or 2''), then get a decent barlow 2x to 4x and see how it goes from there. First thing to check is if you can achieve focus. If you do not have enough travel, you will have to add a diagonal , then the barlow and then the camera but at that point you will be pushing the weight to the limit on your mount (especially if it is windy in your area). You rig will also look a bit messy too but it might do the trick.
Good luck
