Changing the default screenshot filename

It is impossible to deliver an "answer" to this question. You are asking for something that is purely opinion-based. I have friends who use super small fonts and it's very comfortable for them. I also have friends who use larger fonts, and that works best for them. I personally use a variety of font sizes depending on what Mac I am on and what App I'm using. Your best best is to work with whatever font size suits you, despite what anybody else tells you.

There is no "one-size-fits-all" answer to your question.

One approach that's forward compatible and will not get overwritten in an OSX update is to create a script that does ls -l Screen*.png (on the correct directory of course, I'm writing generically) if the list is empty the script exits, if not it uses mv to rename the file (and optionally move it if you also want to, you wouldn't need to since the renamed files won't match Screen*.png (the next time the script runs that is). I would look at using an xargs to pipe the ls to mv. (I'll admit this would take me a minute to cook up, but it's do-able.)

Once you have the script working correctly use cron on a one minute schedule (hint * * * * *). Since most of the time the script will exit after one command this will not use any measurable resources.

Previous answer:

One option would be to use fswatch to rename screenshot files as they are created. This option really has the most forward compatibility as well.

fswatch as the name implies allows you to WATCH for File System changes within a script, and then pipe the output to some other command. In this case probably xargs calling mv to change the name the way you want it to read.

I would reccomend this type of solution, as it is more update proof that modifying the way OSX names the file. (i.e. that is let OSX do it's thing, then just have a script that watches and renames immediately).