If the framing isn't 16" or 24" o.c. it's more difficult to fit batts to a high quality standard.
R19s are arguably the crummiest insulation material in common use today. When compressed to 5.5" (a 2x6 framing bay) it only performs at R1
8. When compressed to 3.5" (a 2x4 framing bay) it performs at R13. In effect it's just a "fluffed" R13 batt- it even weighs the same per square foot,
exactly the same amount of material, but sold at a higher price than R13s. The low density of R19s also makes it more of an air filter than an air-retarder, barely slowing down the rate of any air leaks moving through it.
If it's not too late, R21 fiberglass or R23 rock wool are both far superior products, easier to sculpt-to-perfect-fit using a batt knife than squishy-spoogey R19s.
With non-standard width framing cutting chunks of batt 1/2" longer than the stud bay width (say, if it's 19" edge to edge between studs, not on-center, cut it 19.5") it can fit pretty well with the chunks butted up, and little waste. Cutting them length-wise a half-inch wider than the cavity space works too and it sometimes quicker, but makes more waste. (But putting the shreds between the floor joists and burying them in cellulose works.)
Cellulose is compressed to a fairly high density in the bags it's shipped in. Crumbling by hand doesn't necessarily fluff it up enough to meet spec on R-value (it's well beyond optimal density.) Agitating it in a heavy-duty plastic 5 gallon bucket with a
mud-mixer on a low speed drill can get you there. It's not a super-quick process, but quicker than breaking it up finely by hand. If you have the cover/lid for the bucket (or make one) and drill a hole in a cover/top for the 5 gallon bucket to accommodate the shaft of the drill you can spin it up a bit faster, without filling the whole room with fluff & dust.
Five gallons is about 0.67 cubic feet. Optimal open-blown cellulose is about 1.5 lbs per cubic foot, so it only takes about a pound of chunks fluffed up enough to fill the bucket to hit the right range. Estimate the size of the starting chunk by looking at weigh numbers on the bag using a bit of 5th grade arithmetic and at tape measure, and just slice off chunks according, crudely breaking it into chunks small enough to fit into the bucket. Precision doesn't matter too much, but err to the high side. If it's only 1.2lbs per cubic foot it may settle a bit in the first couple of years, if it's 2lbs per cubic foot it won't settle much at all, and will still perform north of R3.5/inch if thoroughly fluffed.