4 volumes on disk image

That is a great new feature of High Sierra and later when using Apple File System APFS on the boot SSD. Now those small Volumes can share the extra space on the drive rather than having to cut the drive into four fixed pieces to get those four Volumes. That is not the source of your problems.

<<"Sometimes it now runs slow and I get the spinning wheel">>

what Model MacBook pro by year, early mid late, screen size and processor speed?

How much RAM is installed?

DDR3L the L is for Low power, perfect for a Notebook computer, provided it is the right spec.

OWC promises that it IS the right spec, and works in your Mac or your money back. They also say that they will replace those new modules for your lifetime.

I would only go for the genuine Apple modules with only a 1-year warranty to save money, if that is an issue.

When these modules were a little more expensive, I installed ONE 8GB DIMM along with the existing 2GB DIMM and it was much faster and got me out of the Rut of simulating the extra required RAM on the drive. You can install matched pairs if you prefer. Matched pairs will be ever-so-slightly faster, but the difference is so small it can only be detected in a memory test, not in general use.

At one point in the dim distant past you had to work hard to make certain you did not buy "junk memory" that would not even work at all. If you limit yourself to reputable Mac-centric vendors like OWC, there is no longer a threat from "junk memory". OWC or similar Vendors are the experts at what lasts and what does not last, and every new DIMM they sell includes a lifetime warranty, so buy whatever they suggest with confidence.

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Readers who have replaced their original Rotating drives with a comparable-sized SSD drive report, "It's like getting a whole new computer!" Startup and Applications launching are dramatically faster, and many ordinary operations are somewhat faster.

But if opening your computer and replacing parts is new for you, you may be better off doing one upgrade at a time. The SSD swap is a bit more complex, and you can do later -- it whenever you like.

If you do not have a recent backup, your computer is like a ticking Time bomb. You are only one disk failure away from losing EVERYTHING! Drives do not last forever. It is not a question of IF it will fail, only WHEN it will fail.

If you are using another direct-to-disk backup method that you prefer, and you currently have a recent backup, that is great. If not, you should consider using Built-in Time Machine. Take steps to acquire an external drive as soon as possible. If you buy one, a drive 2 to 3 times or larger than your boot drive is preferable for long term trouble-free operation. Do not pay extra for a drive that is fast.  (You can get by for a while with a "found" smaller drive if necessary, but it will eventually become annoying).

Attach your external drive and use

System preferences > Time machine ...

... to turn on Time Machine and specify what drive to store your Backups on.  It may ask to initialize the new drive, and that is as expected.

Time Machine may spend all afternoon making your first full backup. You can continue to do your regular work while it does this. The first Full Backup is by far the biggest backup. After that, it will work quietly and automatically in the background, without interrupting your regular work, and only save the incremental changes.

Time machine's "claim to fame" is that it is the backup that gets done, because it does not ruin performance of the rest of the computer while doing its backup operations. You do not have to set aside a "Special Time" when you only do backups. When you need it, your Time machine Backup is much more likely to be there.

How to use Time Machine to Backup or Restore your Mac:

https://support.apple.com/en-us/ht201250

If you choose to connect your backup drive only from time-to-time, try to do so at least every-other day, at minimum. Otherwise, it may take several hours of computation just to decide what needs to be backed up, before any files are transferred.

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Leroydouglas also wrote a nice article about Boot Clones. This might be something to think about a little further down the line.

 How to create a boot clone

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In the US, these vendors:

http://OWC.net is still the most-recommended supplier here, because they have good return policies and a lab full of Macs. They pride themselves on their service, so they do not sell stuff that is so "difficult" it needs a lot of hand-holding to get working.

OWC sells their own private-label SSD drives, which are pretty good and reasonably priced, and you know they will work in your Mac.

Look for the "My Upgrades" link on their main page, and it will lead you to parts that will work in your specific Mac.

http://DataMemorySystems.com is less often recommended, but very close in service and real-world testing, and often a little cheaper for comparable products with the same terms.

http://crucial.com is often recommended because they have a great track record of supplying stuff that works first time, every time. Personally, I am not as fond of them because they use their database of what sales "stick" rather than a lab full of Macs to obtain their results. Their recommendations tend to be more conservative than others, and they sometimes discontinue parts for older Macs before other sites do.

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All of these vendors have a lookup program where you enter the specs of your computer, and they recommend the parts that will work under these terms:

"It works in your Mac,

or your money Back." (it costs them money to get it wrong, so they make certain they get it right.)

Also, a Lifetime Warranty is standard, and their added vendor stickers on the modules are all you need for proof of purchase if you ever need a return.

Would you like to view a professionally produced video of how to Install memory (or disk drive) in exactly your MacBook Pro 13-in 2012?

https://eshop.macsales.com/installvideos/macbookpro-13-unibody-mid12/

Then you could decide how difficult, and whether you would be up to the challenge.

EDIT: OOPs, I first posted the link for the Retina display model, which unlike yours does NOT have upgradeable RAM.

what market do you buy in?

e.g., US, EU, Asia, Africa, Middle East, or specific country?