Knitting - raglan sleeve too wide. Help?

I am knitting a raglan jumper in the round. I have completed the body (so the raglans are set) and knitted one sleeve, only to find that it is far too wide - so much so that it 'bunches' under the arm.

I have unravelled the sleeve 😢, but am a bit stuck as to how best to narrow the new one. To make things worse, I am using two strands of yarn - a Noro self-striping one and Rowan kidsilk, which is fluffy, so separating them is tricky.

There is the usual underarm cast-on, which comes to 20 stitches on each underarm - 10 on each side of the marker. The pattern says to pick up ten of them in total when starting the sleeves, and I could reduce that, but I don't think by enough to make a big difference without puckering. The remaining stitches are 'live' so I can't pick up fewer there.

The pattern asks for decreases every 8 rows (four per decrease row) so I could do that every 4 rows to narrow the sleeves more quickly, or I could pick them up as directed, with fewer underarm ones, and then knit a row with a lot of decreases so I start the sleeve with fewer. Maybe I could aim for the same number as a smaller size, and follow the pattern for that? I'm not sure how the latter idea would work in practice though.

If anyone has any other ideas, please, I'd be very grateful to hear them. I don't want to have to unravel the rest of the jumper, as I'm pleased with it apart from the sleeves.

The yarn will be aran weight, give or take, so any bulkiness will show.