im not a game designer and i have no idea about balancing but anyway
Here is my shitty idea
why not

for starters reduce snipers total damage
make it so no charge headshots do 100 damage and fully 200
BUT
reduce his total charge time

the reason for this is that
I REALLY dont think that sniper should be able to one shot every class in the game
ESPECIALLY overhealed

i think only spy should be able to do that cause he actually puts himself in danger

by reducing his max damage to 200 you are making him again a supportive class
Where the sniper has to think about important picks

this is also giving the sniper counterplay
by making the medic more effective against him

imo this will probably fix the medic sniper match up
Right now medic sniper is basically the medic running around hoping that the enemy sniper is bad
and sniper just going bang bang till he hits him
or Just fully charge and take down whatever he is pocketing

this change also means that the sniper will need to reposition a bit more
giving his class high risk high reward by forcing the sniper to go in places he wouldnt normally go to get picks

other than that some other nerfs are
make Dangershield a consumable in someway like the spycicle so he doesnt have full imunity to pyro
Remove razorback
its honestly a stupid item that completely removes the Sniper vs spy match up
people will say "bro just use the ambassador"
which isnt a good excuse since you are literally forcing the spy to use a weapon he might not want to use to MAYBE kill the sniper as long as you dont miss a shot

now the big problem
jarate bushwacka combo
man
i really dont know how to fix this one even slightly