Bob,
The SW 20" dob is a 20" f3.9. When viewing the zenith & looking at the design, I would guess that the eyepiece would be no more than 1.8m off the ground. Most of the time it will be lower. Depending upon your height, it may well be more comfortable than a 12" f5 and you won't need a ladder, maybe a step if you are shorter than 1.8m.
At age 61, your maximum pupil dilation is probably around 4.8mm. I and 3 friends are all in the 58-62yo age group and we have all recently measured our pupil dilations at 4.7-4.9mm.
Assuming you, at age 61, are similar, the longest eyepiece focal length you can use for 100% light transmission to your eye is: -
Pupil diam x f ratio =. 4.8 x 3.9 = 19mm.
Using the supplied 28mm LET eyepiece to attain a wider field, the light transmission will be equivalent to a 14 inch diameter scope. So you will want a wide field <19mm eyepiece such as a 17mm Nagler as your killer DSO eyepiece. Of course you can use the longer focal length eyepieces if you accept the light loss. A 31mm Nagler will give you the light transmission equivalent to a 12-13 inch aperture.
Providing you are aware of the interactions between pupil dilation & exit pupil, you can gain great enjoyment from such an instrument as Mike and I and many others do.
I have a whole article about this on another forum: -
https://theskysearchers.com/viewtopi...283294#p280632
cheers
Joe