for the last few months I've had to stop playing ranked whenever i reach above 5, and wait a week for it to drop so I can start playing again.

nobody actually uses matchmaking, and betting that the top 5 people will magically do it at the same time is the weirdest design choice.
if that's not the purpose, then why force MM instead of just letting people continue in the ranked lobby?

the only way to play ranked matches while in platinum tier is to ask someone to come use MM at the same time, which very reliably let's you choose your opponent, hence defeating the purpose.

if i do find someone to play using MM it's usually bronze-silver tier, which again entirely defeats the purpose