To improve accuracy from a Glock you need to improve how tight the barrel locks up with the frame. The only way to ensure that will happen with a replacement barrel is by hand fitting - which means buying a semi-fit barrel and fitting it.
considering you don’t even have the 26 yet, are you sure you will need to improve it’s accuracy? Even with a fitted barrel, we’re not talking about a dramatically different change - even in a Gun when the barrel makes a noticeable improvement at 25yds it’s almost undetectable at shorter distances.
I’ll also add, this is not normally a conversation had re: a G26. This type of upgrade is normally reserved for competition guns or hunting guns. What are you looking to be able to do with your 26?
The upgrade is simply peace of mind. I want to know when I'm shooting that any inaccuracy is a result of me. Also, it is a used gun and not knowing the history of round count and use. Figured a new barrel could improve it
So for pistol barrels, especially modern hardened barrels like what Glock uses, round counts are immaterial. The 9mm doesn’t generate enough pressure to affect the accuracy in a meaningful way short of really extreme round counts - hundreds of thousands.
Glock has an indoor shooting league as part of GSSF. The 26 is easily capable of putting up perfect scores (500/500) if you are. I say that to emphasize - when it comes to Glock pistols, the greatest piece of mind should come from a Glock that’s 100% stock. It’s the only gun I know of that is more reliable in its complete stock configuration than with any customization.
I do strongly recommend though, when buying a used Glock, go ahead and replace all the springs with new factory springs. This will cost less than $20. Springs are THE most important parts to ensure the gun functions and they protect the gun from getting damaged.
Second, find some good sights that you like and upgrade to them. Other than that, you’ve got an excellent gun that should continue to be so for multiple lifetimes.