Is there any way to correct uneven bed temperature?
I had this exact issue with my Qidi X-Plus. After checking with a thermometer there was a spread of 9°C - enough to burn in (always the same) certain areas when printing at higher extremes of temperatures, or causing poor adhesion in other certain areas at lower temps. Very similar scenario too, I noticed this primarily when switching over to glass from the stock magnetic plate.

My solution was quite straightforward - I set the bed temperature on the printer's control panel and preheated it before sending it a print. As long as I left it there for a solid 10 minutes after getting up to temperature it balanced within around 1°C. I observed far more consistent results from print to print.

Now I don't know your exact machine myself, so I don't know if this is possible on the control panel on an Ender 3. However if it's not, as an alternative possibly consider looking up how to stick a pause in the initial gcode in Cura to see if you can achieve the same effect.