Post title kind of says it all. 
To elaborate though, I want to change my default settings so that Blender naturally defaults to Cycles, GPU render, and only 6 cores of the eight in my AMD octocore, with certain pixel tile sizes, etc.
Can this be done without coding something, or is there an add-on which will make it possible? 
Asking because I donāt know how to code, 
and am not really ready to start learning thatā¦ 
Have you tried opening a new default file and making those settings changes and then saving the setup as your new default startup file (Ctrl-U)?
Nope! I had not tried that, because I did not know about it. Thanks a Huge amount, Iām going to give that a try.
Todayās my day to fix lotās of little things, since my good monitor died overnight, and I have to wait until tomorrow for the new one to show up.
Update: CTRL-U does not seem to do anything. 
Iām using 2.90.1, on WIn7x64, maybe that makes a difference? Or maybe thereās something I need to do in order for Ctrl-U to work?
2nd Update: Ctrl U does not seem to work as a hotkey on my install, But I did locate the āsave as default fileā option in the File Menu. Thanks for pointing me in the right direction! 
Sure, maybe that is because I am using 2.79 keymap and the shortcuts are different - but at least you are where you want to be.
Incidentally, if you decide to script this stuff so you can make the settings changes in open files, Blender has a built-in script console that shows you what the script is in python for the buttons and settings you have done, in the āInfo Editorā. You can see it when you go to the Scripting layout and make changes - you can copy and paste the results into a new text file, and then save it/run it and have a simple script for your setup.
Thanks for the tip about the Python console. If I ever lose my mind 
and decide that having a migraine as a result of trying to code 
would be a thing I should do, Iāll check it out, LOL! 
Seriously, though, I appreciate the tip. And yeah, 2.79 vs 2.90 keymap differences would explain things. Once again, thanks so much for pointing me in the right direction, this is already saving me time and frustration. 
See you have it working.
Heres some other ideas/settings for a default start up if your interested.
Iām probably not understanding what I read there, but I donāt think that would really suit my needs. Then again, Iāve only had one dose caffeinated beverage this morning. 
THanks for the recommendation though!
You posted about saving settings for your render as a default start up. That thread has some ideas of settings other people use in there default start upā¦ Just some food for thought. No biggie.
Oh! THanks for the clarification. Iām knd of riding the A320SB ShortBus fore Iāve settled into the studio for the day, and had a third infusion of caffeine. Yeah, a bunch of interesting ideas in that thread. Not all of them compatible with my workflow, though. Still bookmarking the thread, though; knowledge is always good.
In this case I already knew exactly which defaults I wanted to change, I just couldnāt figure out how to save them.
I definitely appreciate the link, though!