Extreme 25 for everything, and my wife uses a slx18.
Then I’ve got an extreme 47 for the odd high stuff.
Can I ask how you make up an extreme 20? Does it still have the insulated base section etc? This would be great to replace my wife’s slx18 to be honest.
You buy the top four sections of an Extreme 25 (Or buy an extreme 25 and put the widest section to one side) and I am happy to spray section 4 with lacquer and use it.
Alternatively buy the top three sections of an extreme 25, then take your wife's SLX18 bottom section, wash it out, buy a new clamp for it and you would have a 19ft 6" pole. (three extreme 25 sections and an slx 18 bottom section.)
The top sections would 'stick out' of the bottom section but it will be no longer than a 25ft Gardiner pole when collapsed.
Fire an email to Gardiner's and Alex will advise.
In fact my bottom section 4 has so far had three lots of 3 extreme 25 top sections added to it with a new bottom clamp each time over the years and it works really well.
An slx/extreme 18 end cap will fit.
Edit: The three sections I replace (usually when one snaps after a couple of years are usually OK for occasional use and I use them as the top three sections of my extreme 47 when they finally give up.)