There is only one option.
Have a open selection to the whole USMC twice a year, everyone is a rifleman right? The selection should be done by each post and qualifications should be posted black and white.
Next the USMC needs to stop talking and produce The best damn Sniper instructors, not the best damn PT smokers, but NCO's who can teach well.
Thats the only way.
Or, send all USMC Snipers to the US Army Sniper school as a train up. Seems to develop a better quality Sniper.
https://taskandpurpose.com/soldiers-marine-scout-sniper-course
Just saying.
All jokes aside, it might be a best idea to have a DOD basic Sniper school that all branches attend, Navy, Army, Marines, Air Force, and Coastguard to set a base line.,