Without knowing which "Deore" RDER you have, one doesn't know the max cog size or amount of chain wrap available.
It might sound "brutal", but it sounds like you bought one step to good of a bike.
The 7.4 lists a 3X9 which would likely fill your needs.
I'm uncertain of which "new" generation of 9 speed RDER's that handle the larger cogs AND Double Rings AND is compatible with your current shifter.
Hopefully one of the knowledgeable ones will step in first.
to add: Your right shifter is a flat bar "road" shifter, shifting a "road" FDER to move the chain on a "road" crank.
IF you wanted a "mountain" triple, you'd also have to change the shifter & FDER to make sure everything functioned right.
My hybrid has been changed from an 8 speed 11-32 / 28-38-48 to a 9 speed 13-25 / 22-32-44. (44 has actually been changed to a 36T, but that's a different episode)
Last edited by Bill Kapaun; 08-30-20 at 07:12 PM.