How can I protect myself from a client not willing to pay final invoice?

Hi Folks! Long time lurker; first time poster.

Not sure if this is the most appropriate subreddit for advice on this specific topic, but I'm hoping enough people here would be able to either advise or direct me.

Just before the New Year, I started freelancing - graphic design, copy, content, and web development. Every job I've received has been word-of-mouth and through networking, and I've had no problems. Until recently.

I've got a client that is 2 months past due on my invoice and now trying to haggle his price down - to the point where (if) he finally pays me, he is just going to pay less than I've billed. It's not for problems with my work mind you, it's problems with the person at his company who hired me over budget. I honestly feel like I can't even push back anymore or he'll just not pay anything.

Now I know my ultimate mistake was not getting a clad contract up front so I can live with getting screwed. A few hundred dollars is a small price for a valuable lesson. However - I would like to get these things in place for the future.

What is the best way to protect myself as an outside freelancer, and how can I implement it legally? Let's focus on webdev for arguments sake.

I am considering the following:

Half up front before beginning. The rest due within 14 calendar days upon delivery of the final product or it will be subject to suspension. Assuming I have root access to the server itself, this is very easy to do - but I want to ensure I do everything legally.

It sucks that I now feel like I need a killswitch, but I'm sure I've seen people mention it before. Any advice as to how to properly communicate this within my original contract, reasonable timelines, and possible blowback?

Cheers!