You only have 8 gb of ram now? Is your MBP one of those where adding ram is less than practical?
The keyboard could just need cleaning for the S key, but I have no idea which keyboard yours has and if it can’t really take keys off. If you can’t, maybe trying blowing air into it - but that could just move any trash into a different area.
One thing you might think about, should you decide to wait longer - but worry that you wait a year, it still doesn’t have a big change, but you don’t want to wait longer. So you buy then and a year later the fancy new ones come out! That sucks, but otoh, would you have wasted any money? I mean, if you spend X amount now or X amount in a year, you still have spent the same money. The only loss is that you wouldn’t have gotten that extra year of using a better computer. But if, in that year before you buy, they make a smaller improvement, you still have a better computer to keep or sell.