I always recommend that people buy the best they can afford, but best is different for different people, and what you can afford needs to include lens prices.
I got into FF in 2017, because I had a really good year. I would not have stretched financially to do it. I wouldn't have done it the year before or this year. And I wouldn't have done it if I couldn't have afforded a full set of FF glass to go with the camera.
My additional justification was the superior ultra wide lenses which you can only use on a FF camera at their intended angle of view. Some people, like me, consider that important for landscapes. Most people use longer lenses for landscapes from 24 mm up, which kills that particular advantage of FF.
Secondarily, I like the low light capability a lot. That only matters for pictures of moving subjects or hand held pictures. You can shoot landscapes hand held, but I recommend using a tripod, so no advantage there, if you use a tripod. I'm a real estate broker and part time real estate photographer, so I shoot promotional and artistic landscapes on a tripod and some hand held when out looking at property. It's great for property tours and party pics.
I am not that big on the shallow DOF, but I have had a very few occasions where it was an advantage. Shallow DOF is not good for landscapes. I shoot mine at F11 on FF or F8 on a crop sensor, so there is no advantage to FF there. It really is nice for some portraits and some shots of discrete inanimate objects.
Don't go full frame if you can't afford the glass that you want. I would much rather have a crop system with everything I want than a FF system that limits my photography. (Many people would disagree.)
Whatever you decide, get a heavy tripod. Nothing, and I mean nothing, no camera, no lens, and no software will improve your landscape photography as much as a heavy tripod. Remember it's all about the light, and the magic happens at dawn and dusk. On a heavy tripod, you're still taking those great shots at base ISO for better IQ than a better camera at high ISO.
On my computer I have an album of my all time favorite shots, and many of them were taken with my old 60D and the excellent EF-S 10-22. I have large prints on my wall from that setup, and wall prints from that setup hang on the walls of clients and friends to this day.