Dianne, the fact that you wrote "Just pulled out my MacBook Pro" and "I haven't even hit the 1/2 way mark on usage" suggests to me that probably you have not used this machine a lot.....and is there any reason that you are now anticipating that you would get more use from it or from a newer machine in the next year or so?
Storage usage on a machine's internal SSD doesn't necessarily indicate the amount of usage that one actually employs with a given machine. For example, right now looking at my 2018 MBP's 1 TB storage states that I have 588.6 GB free. Why is that? Because most of the time fairly soon after I have processed and edited images following a shooting session I then move the files to an external drive and they are no longer on the internal drive at all. But, yes, they did pass through the 2018 and its internal drive for a time and remained on the drive as long as things were still in process, but once I had finished working on the image files, they were shifted off the internal drive to keep it as "lean and mean" as I can, which I think enhances overall performance. The external drives, on the other hand, keep getting more and more filled up!
8 GB RAM used to be the standard and then at some point it gradually shifted to 16 GB RAM. Many of us a long time ago were already using 16 GB RAM and then moved to 32 GB RAM as new machines became available with the possibility of that being an option when purchasing. My 2017 12" MacBook has 16 GB RAM because it was possible at that time to purchase the machine with it, and I did so, looking ahead to the future, even though that machine is not one that I've ever used extensively for editing photos. My 2018 15" MBP machine I purchased with 32 GB RAM right from the get-go because I was intending to use this machine for editing photos and I also was looking ahead to purchasing new camera gear with more resolution than I'd had on any camera in the past, which at that time I had not yet done. I knew that the higher resolution was going to be more demanding on my computer and on its storage system, so planned for that. And, yes, it has indeed proven to be the case that I needed the extra RAM for the challenges presented by my A7R IV and its high-resolution sensor.
Something else to consider about your 2012 MBP: is it still able to handle OS and other software updates or is it a model which has now been declared "Vintage" or "Obsolete" by Apple? I'm not sure but I think both the 13" and the 15" models from 2012 have been declared at least "Vintage" if not "Obsolete." More important than just the updates to the OS, of course, would be the security updates, and I think Apple does keep those going in older machines even when the machine isn't able to work with whatever the current OS is and remains on an older version of the OS. One can keep a machine successfully going with older versions of the OS for quite a while as long as they're also able to continue with security-type updates...... To me that would be something to take into consideration. Nothing wrong with continuing to run a machine on Mojave or Catalina as long as one can also still get those important security updates. Hardware support, though, on an "obsolete" machine is a different matter as far as Apple's Genius Bar is concerned, although other computer repair shops may be able to help out with a problem that has arisen.
The really significant issue in the long run, though, is, just how often in the past year or two have you been using this machine? If it's only something you pull out once in a while then maybe it's not worth bothering with replacing. If it is already behind in OS updates and that is a concern for you, but you really have not been using it anyway, it's probably not worth replacing if it cannot be updated to the current OS (Big Sur) and you don't really have that great a need to use it in general, even with whatever OS it currently does have and cannot progress beyond.
As Morty points out in his post, there is a distinct advantage that some of the older machines have that the new ones don't, and that is the ability to replace various parts as needed. An addition of a faster SSD as an internal drive and/or the ability to fairly easily add more RAM as desired, or to replace an ailing battery, is pretty significant, as these days the new machines are now all locked down pretty tightly so that users cannot make even seemingly simple changes themselves. If the RAM or the SSD in my M1 13" MBP goes on the fritz, no way I can simply open up that machine and swap things out-and-in.....I'd have to take the machine to the Apple Genius Bar to let them deal with the situation, as everything is soldered into place.
Bottom line is how much have you used this machine in the past and how much do you anticipate using it or a replacement in the future? Would it be worth tinkering in and adding more RAM and perhaps a new battery if needed, or just overall better to go with a new machine in the first place? How important is it to you to be using the current version of Apple's OS? If not important, again that makes the decision a little easier in terms of simply holding on to what you've already got rather than plunking down a bunch of money on a new machine which also may suffer the fate of spending most of its time in a closet or drawer somewhere rather than being actively used on a daily basis.....