it's hard to hurt the plants of cuttings with H2o2. Not knowing what you had available, I left it open.
It's been so long since I read instructions for that stuff.
If you can get the strong stuff from a hydro store (27% I believe) then dillute it a lot. I usually like it strong enough where it will still burn my skin (turn it white) but slowly.
If you are working with the stuff from the drug store (3%) run it straight from the bottle. You definitely won't hurt the plants with it.
Fungus gnats suck. Very stubborn to get rid of.
First, understand the conditions that bring them:
Soggy root balls
Dead plant matter laying around
standing water puddles
If you have plants struggling to root in these conditions and you start throwing heavy oil in the root zone like neem products, it can (in my experience) make things worse for the plant while not really bothering the gnats.
I've tried everything to get rid of them once they appear, and never really won. Lost some plants, but by fighting, some survived to harvest. After harvest, cleaned the room thoroughly and never made the mistake of letting those conditions that they breed in exist again.
It's been a few years since I've had to deal with them.
When I had them, the first thing I did was stop overwatering, and clean the crap out of everything.
No more dead leaves laying around. No puddles of standing nutes from clogged drains. Clean clean clean.
After that, I let the root balls dry out, pulled them out of the pots and removed any dead/infected areas of roots with a knife. The decomposing roots and waste from the pests can create very unhealthy conditions for the rest of the root zone.
Then, re-pot, first putting a good layer of Diatomaceous Earth on the bottom of the pot before adding new medium, and dust the root ball with it. Once re-potted, cover the top of the soil with Diatomaceous Earth.
I found a liquid that contained Bacillus thuringienis israelensis (the same stuff as in Mosquito Dunks product) and started adding that to my nutrients (hydro).
Just checked around and I can't find it for sale anymore, but the mosquito dunks people have a product called mosquito bits that might work better for you as top dressing or even mixed with new soil:
Summit Chemical Company | Mosquito and Insect Control Products – Mosquito Bits®
I would avoid putting any organic stuff like that in your hydro res. Made an awful mess and I suspect led to swings in Ph.
Regularly scrape any loose, dead stuff off the top of your pots and reapply diatomaceous earth and fresh mosquito bits.
good luck.