Lars,
Belt mod (with 4:1 or 5:1 GT2 (12 & 60) belts & pulleys) fits nicely inside EQ6 enclosure (4mm belt width is quite adequate), while for T2.5 you will have to remove the part of housing to fit the larger pulley (it will protrude out of it - some people did that, though). It is also easy(er) to adjust transmission ratio (by varying teet number on pulleys), and Banggood has large number of various timing belt lengts in stock, cost is couple of AUD. And 3D-printing of 47-teeth pulley is a viable option.
Also, OnSTEP electronics is actually very affordable/low cost solution, so it make sense to sell existing controller and maybe recover a significant part of the investment in new electronics.
As to stepper with planetary gearbox, there will be backlash (no backlash is present with timing belt transmission/reduction). This motor is 200steps/rev.. so you will have to use 32 /usteps or even 64.
Also, any gearbox will be louder compared to timing belt transmission. This motor assembly may not fit inside EQ6 either.. External placement is not so good for mount carrying around as it could be easily damaged. And if it does fit inside, you will need belt transmission to couple it to worm anyway...
Have a look at
this webpage, it is similar to what I have done (I went for 5:1 belt transmission, 60 & 12 GT2, and I did not make those large holes in the aluminium enclosure)