You can choose to delete them or have them still stored locally. I don't recommend going full Google Photos as they do compress some stuff even when you choose "Original Quality" as your backup option.
Through your post, its clear that you would like all your files on your device at all times so just go for the 128GB option. Resell your X while it still holds value. The screen PPI topic is overblown, unless you spend your days pixel peeping all the time, you won't notice a difference. The wide angle portrait mode is really nice compared to dual lens ones but remember, default camera app limits portrait on human faces only.
Resale value is definitely not going to be as good as the true flagships, but it's easier to keep it in good condition since the aluminium doesn't scratch as easily as stainless steel. It is a lot easier to handle as compared to my previous 7 Plus, as long as you don't get a bulky case. I do miss 3D touch a lot to be honest. It is a feature that I got used to but didn't realise I'd miss it till its gone.
Though ultimately I think your use case warrants an iPad since it has a bigger canvas for you to do editing etc