I shoot mixed arrows all the time at 15-20 yards.
all are spined OK for my bow, but have disffernt materials (aluminum, cedar, ash, lamintaed birch) and vary 1.5 inches in length, and point weights from 100gr to 140 gr.
Ok, If I shoot with correct form, I find these arrows grouping in 2 inches or less at 20 yards.
Easily group well enough to keep 'em on a tennis ball.
But, for a long time I thought I had over-spined arrow (in spite of the charts and recommendation) because of shooting left.
It was me bow-arm. Not keeping it on target 'til the arrow hits. Follow through. If I don't focus to hold that bow arm on target 'til the arrow hits, I shoot left. Maybe 18 inches at 20 yards!
Even a brief lapse can push you left on release as you let that bow arm cast left.
OK, I fix it by keeping my eye-mental focus on target "own the target" 'til the arrow hits. Just follow it in, and my bow arm stays on target without me trying to hold it still. It's not about a stiff or rigid bow arm. It's about keeping all focus, including the bow arm on the spot as a follow through. Mostly mental focus.
Try that, and see if the arrows spine better. Provided they are spined right to start with.