Actually, I am still somewhat unclear on this part. Were you really only using 14.5 GB? Cuz that seems too low, as it's less than a basic install of macOS plus MS Office. And how much storage do you need for your photos? Your post suggests you may have around 100 GB of photos or something like that.
If you use iCloud storage at the 200 GB tier, that's $3 per month in the US. With that you can keep much of your SSD storage space free, and have real-time backups for your iPhone, with all the originals for your Photos Library in the cloud, accessible anytime from all your machines.
However, if you don't want to pay for iCloud, and your Photos Library alone is already 100 GB now and you want to keep all the originals local on your MacBook Air, then getting a 256 GB SSD might be too tight after a few years. In that case, a 512 GB SSD might come in handy for some breathing room.