What should I do to secure my Steam account after I redeemed a Steam wallet code sent by another user? As that user can now possibly access my account

I redeemed a Steam wallet code that was sent to me by another Steam user. Before I did this, I asked a different question in this link in Arqade - Stack Exchange. I asked if my computer/account could get hacked if I redeem a Steam wallet code sent to me by another user. The answer to that question by Joseph Richardson shows that "One of the methods of account recovery is providing to the Steam support team a game key code or wallet funds code that has previously been redeemed on the account (as per here)" so "If you redeem the code then the scammer can provide that code to Steam support, as well as proof of purchase, and gain access to your account." Well I have already redeemed the Steam wallet code so my account can be accessed; What should I do now to secure my account?

You should contact support and let them know.

This kind of trick will usually take time, since that historical record will be associated with you permanently. The hacker can wait until you have not used your account for a while, then claim it and re-sell it before you know it got compromised.

Let support know that you redeemed a code from an unknown 3rd party, and, if possible, reverse that redemption (you'll lose the funds, but better than the whole account).

Next time, do not accept free stuff from random people. There are no free lunches.

If you want to scam the scammer, make a new account, claim the code, and gift yourself a purchase. Never associate unknown redemptions on your main account.