PLA as a raw material is really cheap - you can get it for as little as a few dollars a kilo - and a lot of the cost of a filament roll is just making the spool, packaging, and shipping it. In the end, the price difference between 250g and 1kg of PLA on a roll is rather tiny. Goes the other way too, you can buy 5kg rolls that come down to just $10-15/kg.
The same reason why a can of cola is so expensive compared to a big bottle.
Also check if you have TMC drivers and if they have Stealthchop enabled, it can cause these types of issues. Especially for the extruder motor.
Extrusion variance and bad tolerance filament will also exhibit the exact same looking issues, not just poor Z alignment.
Just to be sure, you have z-hop disabled? That can cause all kinds of issues if the gantry isn’t absolutely perfect.
Another thing would be making sure all variable line width printing options are disabled, I.e ones that try to fill gaps with thicker lines or print thinner lines faster to stretch the filament.
And finally, melt temperature oscillations, making sure every feature - outer/inner walls and infill - are printed at the exact same, slower, speed.
As for actual z issues, if the bed/gantry isn’t freely moving and has even a slight bind on the screws/axis, it can miss microsteps. One rather harsh troubleshooting tip is to reduce z-microstepping to a lower value, as that gives it more torque per step and usually you do not need the z to move anywhere near as gradually as some microstepping setups theoretically might allow.
That “slicer file is public domain” part would probably not be valid as benchy has a “no derivatives” licence:
If you remix, transform, or build upon the material, you may not distribute the modified material.
That overrides the requirement of “share-alike”, meaning you have to use the same license for remixed or derived content, as you aren’t allowed to release any in the first place.
Unless the creator has received permission from them, of course - that overrides all licences.
You did try drying it in a dehydrator before you did that, right?
Because I have spools almost that old, they print fine after a night in there. Brittle as dry spaghetti if I don’t though.