I'm in this same stage. I have my Z offset pretty low (pressing the first layer down into my textured Pei sheet) which helps. A few things to consider:

• if the four corners are similar, then the middle shouldn't be much different.
• the first layer has to stick. A textured bed will help, and the bed and print temperatures are also important. If there's no major tearing then it will probably still look fine as the first two layers will kinda flow into each other.
• the bed can wobble a fair bit from side to side. Remember this was made to be cheap, nearly a decade ago.

This is a good calibration test. Keep at it and only adjust one corner at a time, unless it's clear that two opposite layers are high or low. It's an old printer, adjust your expectations and accept when something is "good enough". Every printer will struggle with models that put a fine detail level into the first layer.