Sd cards are really only for personal pictures and videos, and other media.
So any of this that is on device memory should go to the card.
As far as the device memory, Samsung seems to enjoy hogging vast amounts of that for the OS.
I am not familiar with the device, so bear with me if yours differs from the following things to try.
Settings
Storage
Internal shared storage
Other apps
overflow, the 3 dot menu, upper right
Show system
overflow again
Sort by size
The largest apps are on top.
Select and delete the caches, and see how much space you get.
Some apps are real pigs, as you will discover.
Especially social media apps.
These are best to uninstall or disable, and then access them with your browser instead.
Google is like a hog pen of apps.
There are multiple interlocked apps that all can get large.
If you can live without Google, then by all means do so.
If you want further assistance with these Google apps, I can help, but it can get pretty involved sometimes.
Without the device in hand to see results and testing it could be pretty difficult.
You have reached a point where either you have put too much stuff on the device, or else updates to built in apps and the OS have made them become too large.
The OS cannot be easily reverted back, but apps can.
Functionality of these apps may or may not be hampered.
It all depends on how much time and effort you are willing to put into an older device.
Keep in mind the age of the battery.
Is it easy to replace?
A new unlocked Moto e has double the internal memory and can be had for $120 USD.
But I have very often been broke and only able to use the most sparse of devices.
So if there is a want and a way to get that thing working correctly again, I will try to help.
When you see the percentage of internal memory used, what does it say?
A good rule of thumb is to use no more than 70%.
This allows for apps to expand as you use them- system apps do so as well.
They can expand and contract with use, but need room to do so in order to function correctly.
One thing that I have noticed is that when I used to overfill a device, certain apps would carry almost no cache at all.
Now that I leave much more open space, these same apps can get rather large caches.
So actually, I wound up having to cut back even more on how many apps I have on a device.
Until very recently I too had 16GB devices, and even a couple 8GB devices as well.
So I totally understand what you are going through with these lack of memory issues.
The quick and easy solution is to just get a device with more memory, but if you cannot afford that or if you wind up overfilling the new device as well- then you are back to square one.
I would save money for a larger capacity device while learning how to make a small amount of memory work for you.
That way, when you can afford that better device, you will have the knowlege to keep it functioning at peak capacity without worry of overfilling it.