Ipad pro 2020

If you're not going to install many apps and have many large videos / photos etc. 128GB may be enough for you. Bear in mind you can use external USB storage with a supported filesystem and doesn't require power beyond the iPad's ability (otherwise a powered hub may be needed) and you can easily access / store using online storage services such as iCloud, OneDrive and also SMB shares (NAS drives or computers) etc.

https://support.apple.com/en-us/HT206481

Of course any device could fill up to capacity if not careful and selective... I suppose the saving could contribute towards a keyboard / mouse / touch pad / whatever?

Hello,

Only you will be able to answer this question as you are the only one who knows what your data consumption needs are. Most likely over time you will feel constrained with the 128 GB option. If you look at any of your other devices you should be able to see what their storage capacity is and how much of that you have used.