You wouldn’t always need to keep it in there, you could un-mount, remove it, use the SD slot for something else briefly then re-insert the “additional storage”.
To be fair, it’s not the most elegant solution, replacing the internal drive as others have suggested is probably the ideal approach for a long term solution. If you don’t like the idea of the MBP being opened up or just want to keep it ticking over till a complete new machine is viable, then this might buy you that time. The other issue with it is I doubt the speed will be that fast either. If you’re just using it for storage rather than a working area it might suffice though.
You might also find this link useful, you can have multiple photo libraries, doesn’t look like you can easily move photos between them without doing an export & import process but it could allow you to have an “archive” library on external media and a “current” library on the internal drive:
Do note the section about moving photos to an external drive, that seems to suggest that you can only have one library sync’ing with iCloud.
When you say you want to use the MBP ‘out the house’ is that well away from the house or in the garden & in still in wifi range? Is some sort of NAS storage an option?