My eyesight is getting worse as i get older and i feel for you terribly!
I think yellow targets are usually easy enough to find for the average eyesight, sometimes they tend to blend in if there are a lot of Autumn colours about or similar.
We use road paint, as do most clubs as it is cheap, occasionally i have had a much brighter yellow delivered by mistake or just different batch.
I will be honest and say i don't like any bright yellows i have used as i get a lot of glare back in my scope, particularly when the target is close.
A minor thing compared with your problem i know but it affects my rangefinding to some degree..
I believe a bright large number would be the way forward..
Not being sarcastic and i'm not sure if it would be allowed let alone work but have you considered a REALLY bright torch for those targets in woodland...something really bright may just help you find the target if your partner wiggles it around on the plate??...would that be legal even??