On my two touring bikes, I use a 52/42/24 front and 11/12/14/16/18/21/26/32 Sram eight speed rear. That gives me one and a half step gearing with the outer (52t) and middle (42t) chainrings and several very low gears for climbing with the 24t granny chainring.
Changing from the 30t to a 24t chainring would give you lower gears at low cost. The upshift from the 24t to 42t chainring is not very smooth, but I rarely use 24t chainring so it is not a common shift. My front shifter is friction bar end shifter.
Yes, if you use a MTB cassette, you would likely need a long cage and a few more chain links. My XT derailleur cage is not long enough to take up the slack when I use the 24t front and 11t or 12t rear, but I never use those gears anyway.
