Which is the most stable os version for Macbook Air 2017?

You can't really determine which OS is "more stable" since other's systems/OS may be stable for them but unstable for you depending on what software you do/don't have installed. Antivirus software (unnecessary on Macs) or apps like "Mac cleaners" can be a huge drain on the processor.

If as you state, Monterey feels "slow", it may be that updating to the newest OS (Ventura) may cause your computer to feel even slower since newer OS's are usually more processor dependent and are doing more in the background.

Unfortunately, you cannot add more RAM to your MacBook as it's soldered directly to the mainboard.

Have you tried reinstalling the OS on your MacBook and seeing if that alleviates the "slowness"? This will get rid of any errant software that may be installed on your Mac and allow you a "clean slate". That may be an option for you to try since we fellow users have no clue as to what exactly is installed & what processes are running in the background.

I looked at a friend's 2016 MBA the other day with your same specs. His battery was draining quickly and the laptop ran slow. Right away I noticed that he kept a webmail page and more than 8 apps open all the time. Shutting down some open apps helped with speed and battery life. Since you are a developer, I'm sure you already know this. I'm using Ventura on a 2017 iMac with the same memory but a faster processor and feel it runs the faster than the original OS.