I’m not an advocate of additives for a vehicle that one intends to keep for a long time. The best solution, since you’re admittedly not mechanics, is to first carefully follow the directions in your owner’s manual and be sure it has the correct amount of fluid, then take it to a reputable independent tranny shop for a look-see.
If the tranny fluid is low and adding fluid stops the shudder, I still recommend getting it to a shop. And when you check the fluid level follow your owner’s manual instructions TO THE LETTER. Different vehicles use different protocols for this. Toyota typically requires that the tranny be at operating temperature, the engine running, and the shifter be cycled through the gears, but do not take my word for it… check the manual.