What would you consider as "a large quantity" of rare goods? Using
https://inara.cz/elite/commodities-rare/ I never had a problem in filling up my trading Cobra Mk.V. Merits get a boost if you can sell the rares at a large profit. Other things that went well (at my scale) were scanning of Megaships (in Reinforcement, Acquisition or Undermining systems), selling exploration data, collecting assorted wreckage junk from undermining systems (and turning them in at friendly systems in range, I haven't yet checked what junk collecting in acquisition or reinforcement systems brings in - or how the merits vary with handing in in various systems). Escape pods don't seem to count towards merits.
Can't give you the numbers, though, as I'm not on my gaming PC right now. Megaships yield 500...700 merits per scan (some researchers have two scan points), a load of power wreckage junk is at least in the same order of magnitude, as is a hold full (64 tons, ok) of high profit rares. Leesti is a nice source, as you can get two high volume rares there - as long as your destination system buys Leestian Evil Juice....
Edit: regarding range - my cobra does a bit over 200 ly on a full hold without refuelling, so I can get maximum rares profit.
Edit again: plundering 4 or 5 power wreckage POIs in CD-25 636, bringing the haul back to BD-22 286. 6 Black Boxes, 1108 Merits - 17 Wreckage Components, 903 Merits - 19 occupied escape pods, no merits but half a mil in credits - 3 damaged escape pods, no merits and a bit of loose change.