Understood.
There are lots of options.
What ideas do you have now?
What are your client's requirements? Can you describe the client's environment (without violating privacy concerns.)
Basic questions: how much data needs to be backed up and how often?
What resources (hardware/software/communications) are currently available to the client?
If data is lost how fast must recovery be? Is the data sensitive - are there security concerns?
What is the budget? Up front costs and then follow-on costs?
Do you want to support the resulting end solution? Do you want to be liable for either the solution(s) and/or support.
Many questions to be asked and answered.
Simply start writing things down as they come to mind.
Research accordingly. Look for possible answers.
You have some familarity with Google Drive. Fair enough. Do you use full, differential, or incremental backups. Which one would be suitable for your client's requirements?
It would be all too easy for others to respond (based their knowledge) and just say use Google Drive. Or others may say use OneDrive, or Apple Drive, etc...
Consider that an in-house solution may be more suitable. No reliance on outside resources. The cloud is not and should not be considered an absolute or final solution. (Although some cloud providers would like us all to believe so.)
You will help yourself and your client to delve into the details regarding the requirements and potential solutions.
Create an initial plan and a couple of optional plans. Your "n point(s)". The plan will be imperfect and include various trade-offs. What risks are there?
Very likely that there will be changes. The first plan is rarely what gets implemented. May prove completely wrong for one reason or another.
You have to start somewhere.
Plan 1: Pros and cons.
Plan 2: Pros and cons.
Etc..
Overall, you must be able to present specific technical questions accompanied by what you believe and understand to be viable answers. And why.
Much work and effort required.