Clip studio paint is made specifically for making comics, so you get a lot of tools that make making comics easier in that program than others.
-panelling tools
-word bubble tools
-screen tone tools
In addition to most tools that you can find in photoshop or sai. The only thing I miss from photoshop is the ability to edit the shape and perspective of words I intend to use as sound effects.
As for the first two, not sure about Sai because I have not had to use it in ages. While I have no idea where you can get photoshop without subscription, (honestly that's why I ended up switching from phtoshop to CSP in the first place. I did not have a credit card for subscription services, but I had paypal. Adobe did not accept paypal at the time, not sure if that has changed) And if you can find somewhere where you can get it for a permanent licence... well those used to be in in thousand range of $$$ when photoshop sold before the sub service. So that may not be a reasonable option depending on your budget.