Developer options settings has a setting to set default USB configuration. Its name could vary depending upon your ROM, so try looking for any setting having the name "USB configuration" under Developer options. Click on that, and choose the option which has "USB tethering" or "RNDIS" written.
From than on, whenever you would plug the device with a PC using USB cable and unlock the device, if not unlocked already, USB tethering would automatically be enabled. I tested this on a OnePlus 6 running Android 10 and the settings change persisted across multiple plugging/unplugging and reboots.
Developer options settings are by-default hidden though. To enable the access to them, you would need to follow certain steps. Google's instruction per "Configure on-device developer options" are:
On Android 4.1 and lower, the Developer options screen is available by default. On Android 4.2 and higher, you must enable this screen. To enable developer options, tap the Build Number option 7 times. You can find this option in one of the following locations, depending on your Android version:
- Android 9 (API level 28) and higher: Settings > About Phone > Build Number
- Android 8.0.0 (API level 26) and Android 8.1.0 (API level 26): Settings > System > About Phone > Build Number
- Android 7.1 (API level 25) and lower: Settings > About Phone > Build Number
Note: instructions for your ROM may differ if the aforesaid instructions from Google fail to work to enable Developer options. Your OEM or third-party websites are better alternatives to search for the solution in that case.