If your chain isn't totally stretched out you can shorten it, skip the RD and try it on different combos right now.

some of those kits come with 3 cogs to try. Most cogs are too skinny, though, get a thick one after you find your ratio. Thin ones chew up freehub bodies.

i run 1/8" chain even on 3/32 cog. Plenty of $10 options.

In addition to halflinks,
Eehouse has a handy magic gear calculator.
Rennen Decimal Chainrings can help take up slack, but kinda spensive and not recommended for heavy riders.