3DPrinting

I know it's not as popular anymore, but this is why we calibrate our printers. [(https://ellis3dp.com/Print-Tuning-Guide/articles/index_tuning.html)] This is an excellent guide to getting your printer to work to near perfection if you do all the tests. But it does take time and effort to learn and do.

If you switch to Orca, they have a pretty decent calibration suite built in. It's not as rigorous as Ellis, but good enough for most things.

Might not be your issue, butI had issues like this on my ender 3 and I eventually replaced the hotend and they went away. It was like the old hotend just didn't have a consistent flow and there was some sort of clog or heating issue

You could try increasing 'Infill/perimeters overlap'. Its helped me before. https://forum.prusa3d.com/forum/prusaslicer/gap-between-solid-infill-and-perimeters/

The right flowrate for dimensional accuracy is likely not the right flowrate to end up with solid parts where internal lines are properly smushed together. The sides of a 3D printed object aren't flat, so if you adjust the flow rate so that the bits that stick out the most are exactly where you asked the slicer to put the edges of the wall, you end up with your internal lines just barely touching each other instead of properly bonded. You want to tune your flow rate to get solid parts when you ask for them, set the line width a little wider than your nozzle to give space for material to flow outwards (which happens whether or not you want it to, but things work better if you tell the slicer it's going to happen), and then when you've got a part that really needs the dimensional accuracy to be right, maybe temporarily use Orca Slicer instead to get its precise wall feature.

If it's in corners only tune Pressure Advance, but as this is on the first layer it's probably only the first layer, which is generally different from subsequent layers. Check the same thing on a higher layer to make sure that there really is a problem.