Hi pgp566, welcome to the forum
Avast updates are incremental, and rather tiny, typically a few Kb. It is going to have virtually no affect on your download allowance.
However, if you want to disable it, go to the program settings and select "manual" (where the cursor is on the illustration) for both program and VPS updating.
The program updates, if you are not using the latest version, would definitely be large. As stated, the VPS updates tiny. If the program is seriously out of date, the VPS updates may not work, anyway.
The current program version is 4.8.1335. If you don't have that version, sooner or later you will have to update it.
Avast is designed to be run in the background, so there is no "exit" option. You can "stop on access protection" by right clicking the tray icon, and making the appropriate choice. The only services remaining will be ashservice and ashdisp. Between them they will typically use only 30Mb or so. Nothing will be scanned, and therefore nothing should be interfering with the running of the camera.
If that is still unacceptable, I don't think there is any other option but to (a) uninstall the program for astronomy sessions, or (b) stop the services via the taskmanager or via services. To do that you would have to first go to program settings>troubleshooting and tick the box that says "disable self protection".
Malware Bytes is not an antivirus, it's an antispyware.
What is your RAM and CPU? The lack of RAM is more likely to be the core problem that the Avast service.