After "up-grading"to the Android Marshmallow O/S (another big mistake) I learned that all apps and apps data and cache must be stored in the internal memory without the option of storing some of it on the SD card.
Note that this was written off the top of my head with no recent research or "in the trenches" support experience.
This may be the case in your case but I don't believe it's generally true.
I have come across some Samsung phones (Duos?) released several years ago that couldn't move apps to the SD card but most phones can.
As I recall from the research I did when my phone (Motorola Moto G Gen 2 dual SIM) offered Marshmallow and suggested that I format my SD card (Eek!) I found that if you wanted to be able to move movable apps to the SD card you had to format the card.
BACKUP all of your data on the SD card - photos, movies, ebooks, audiobooks, music - before you initiate the format.
You don't have to worry about backing up your apps but you should move the ones you have enough storage for back to internal memory. However, if you acquired them via the Play Store, you can redownload them. I don't think there is a way to do them all automatically. You have to select each one individually.
This is a good thing as it will let you decide whether you really want the app whereas you may have been loathe to delete them before.
Just like Windows PCs need to be COLD booted frequently, not just Sleeped or warm booted, Android phones should be switched off regularly to clear the garbage.
And just like Windows should be bare metal reinstalled every few months, though I don't know whether the new Refresh OS feature in Win 10 obviates the need for going the Full Monty, Android phones should be factory reset every few months.
My feeling is that to to get the full benefit of the upgrade to Marshmallow you should do a factory reset at the same time as you format the SD card.
On most phones you initiate a factory reset by holding down the Power button and the Volume Up or Down buttons. With Samsung phones you have to press a third button simultaneously - Home I think. A menu is displayed before the reset is triggered so it is (should be) safe to test these key combinations.
I believe the camera app can be configured to save photos to SD card so they don't fill up your internal storage.
If you need to find all the existing data in internal memory - photos, movies, ebooks, audiobooks, music - so you can move them to the SD card I recommend the ES File Manager app.
The screen under Settings, Apps, specific app has changed so that even if an app is movable it no longer displays a Move to SD card button. You have to tap Storage, that looks like a heading with no indication that it is tapable. You then have to tap Change to move the App to the SD card.
If you go in via Storage & USB, Internal storage, Apps, specific app if an app is movable it displays a Change button.
It is total pain to have to look at each app to see if it is movable. Install AppMgr III and it will scan all of your apps and tell you which ones are movable. In earlier versions of Android AppMgr could automatically move all movable apps but due to changes in Android security this no longer possible. You have to initiate the move for each app manually.
AppMgr also scans the cache usage of each app and gives you the option of clearing the cache for individual apps or, at least in this instance, AppMgr can clear all caches in one go.
When an app is updated it is automatically moved back to internal memory.
I think AppMgr continues to run in the background if you have run it after a reboot and it will tell you when an app can be moved to the SD card. Maybe you have to run it yourself.