That all depends upon whether what you want to do with the 6D Mark II can be done using an EF 50mm f/1.8 II or EF 50mm f/1.8 STM lens or not. It also depends upon whether you can do the same thing with your current APS-C Canon body and the EF 35mm f/2 or similar lens.
It probably also depends upon what other things you might want to do photographically that won't be possible, at least in the near term future, if you spend the money on a 6D Mark II instead.
Choosing the right or best camera to do a specific task is part of what it means to be a competent photographer. The more knowledge and experience one has, the better one should be able to consider which of the tools available will do the job. Sometimes one has to settle for a tool that will do it good enough. At other times one might have the option to use the best available tool to get the best results (currently) possible for a particular project.
In order to properly answer the question, Would upgrading to a full frame Camera be worthwhile with just one lens (for you), one needs to know what it is you wish to accomplish with the EOS 6D Mark II and a 50mm f/1.8 lens and how that camera is a suitable tool for the task compared to the current tools you have available.
Since you haven't told us...
- What bodies and lenses you are currently using to shoot
- What kinds of photographs you are making now
- Whether your current gear can get acceptable results for those kinds of photos
- What other kinds of photographs you wish to make in the future that your current gear might or might not allow
... it is very difficult to definitely answer your question.
For a general look at the differences between cameras with smaller and larger sensors, please see this answer to Is there really a difference between crop and FF sensors anymore in regards to pic quality?
Addendum following major additions to the question:
I don't have money to buy both the body and the lens at the same time, but later on I'd like to get the wide angle EF L 16-35 f/4.
There's no "right" order to buy two pieces of equipment that must be spread over time due to financial constraints. However, you might find the EF 16-35mm f/4 L IS more useful with your current APS-C camera than you'll find the FF 6D Mark II with only a 50mm prime lens in the interim. And the lens is currently about $600 cheaper than the body, at least here in the U.S.
Both the EF 16-35mm f/4 L IS and the EOS 6D Mark II are great values in terms of price/performance ratio. Other than a very few minor points, the 6D Mark II (currently selling at $1,600 from authorized Canon dealers in the U.S.) is a camera equal to the previous generation 5D Mark III that debuted at $3,400! The EF 16-35mm f/4 L IS is also a great optical performer for a $1,000 lens. But upgrading a camera or lens will not make you a better photographer. It might allow you to use more of the skill, knowledge, and experience you have.
Gear with higher capabilities can certainly help. But a better camera won't make you a better photographer. It will just allow you to use more of the skill, knowledge, and experience you've picked up along the way. Part of that experience and knowledge contributes to the ability to pick the best tool for the job from among the options one has available.
The key to being an outstanding photographer is not having the best equipment in your hands. It is knowing the equipment you have well enough to know what it will and will not allow you to do and then working within those parameters to get images that the technical capabilities of the gear at your disposal will allow you to take.
Just one prime lens is limiting, but I thought that buying a FF body
now would "commit" me to high image quality photography, and next year
I would buy that wide angle lens to really switch to full frame.
You don't need a FF body to commit to producing images of high quality. You need to commit to doing the proper things in terms of composition, lighting, technique, and post-processing that are required to produce high quality images.
I know FF is not a solution for every problem with image quality, but 6D II pictures are just crisper, sharper etc. not to mention low light photography.
Crisper, sharper, etc. than what, exactly?
You haven't told us what camera or lenses you are currently using, other than one of several possible 50mm f/1.8 lenses Canon has produced over the years. Those 50/1.8 lenses are great for what they are at the price point they are sold, but in terms of wide aperture prime lenses, they're not exactly world beaters.
"Crisper, sharper images" are a result of many things. Among them are most importantly good light, good lenses, and good technique using them.
Another possibility is to wait another year for the EOS R price to
come down and then invest in the future-proof RF system, especially
since I could then use my EF-S lenses.
No piece of photographic gear is "future proof." All of it will eventually be replaced by something better or cheaper to make. The question, though, that needs to be answered is, "What is available now that will allow you to take the pictures you want to take now?" If you decide to wait until the next, better, new thing is released, you'll never take a single photo because there will always be another "new and improved" camera or lens on the horizon.
Be aware that using EF-S lenses on a full frame EOS R will crop the images and you'll lose most of the 30.4 MP resolution as only the center 11.9 MP will be used for images taken with an EF-S lens.