You don't "redeem" status points. You should have already received a lounge invitation when you reached 200 status points. It should have a 1-year validity from the time you hit 200 status points. All lounge invitations expiring in 2023 have their expiry moved to 31DEC2023.

Once you hit 300 status points, your membership will automatically be upgraded to SL, valid for 12 months (until the end of that month, so it may be anywhere between "12 months" and "13 months minus 1 day"). If you have an upcoming trip in Sep., you should be able to strategically buy the right fare and subclass to take you over the limit. Use your lounge pass now because once you are SL, you get lounge access anyway (unless you want to save the lounge invitation for your companion). You may want to consider not including your membership on some of the flights (but not the one you will use lounge invitation for), so you can delay the status points crediting, so your SL membership can start later and end later. You can retro-claim your status points in early Dec. Despite the recent devaluation, CX SL is still a very valuable entry-level élite status, offering lounge access when flying with CX.