Fretless intonation & 5-string question

I recently started playing fretless. I often check against an open string or harmonic to work on my intonation. This works well, leading to a better ear, as I need to know which interval I'm checking. It's also helped me better understand what I'm playing in a harmonic context. And it just makes me listen to the sound I'm producing more.
So a great experience so far. I feel like it's making me a better player. I highly recommend.

My problem is: Above the 12th "fret" I find it quite hard to get the intonation reliably correct. Once my finger width gets over 1/2 the fret "width" it get's pretty ugly (at least for me). A very little movement is a big variation.

So I tend to re-phrase things to be lower on the neck when possible (also a great forced learning thing). Which got me thinking. If I had a 5-string w/ a high C, most of the things that must be played up the neck I could move back down 5 frets and play on the C string.

Is this a reasonable thing to do? Or will it just be a different kind of bad on the C string?
Or should I just suck it up and sort myself out on the 4?
 
Common problem with any instrument. The higher you go, the closer together the notes are because of how the harmonic series works. Even with trumpet, the fingerings start to not matter once you get well above the staff. You’ll need to train your ears/brain/fingers to find those notes.

best thing to do is practice very simple Melodies/lines/scales you can play with your eyes closed down low then repeat them up high, listening for when the notes are right. Do a little every time you practice. It will take a while but will be worth it.

I’d give it some work before switching to a high c 5. The wider neck and extra string may or may not outweigh the benefit.
 
I recently started playing fretless. I often check against an open string or harmonic to work on my intonation. This works well, leading to a better ear, as I need to know which interval I'm checking. It's also helped me better understand what I'm playing in a harmonic context. And it just makes me listen to the sound I'm producing more.
So a great experience so far. I feel like it's making me a better player. I highly recommend.

My problem is: Above the 12th "fret" I find it quite hard to get the intonation reliably correct. Once my finger width gets over 1/2 the fret "width" it get's pretty ugly (at least for me). A very little movement is a big variation.

So I tend to re-phrase things to be lower on the neck when possible (also a great forced learning thing). Which got me thinking. If I had a 5-string w/ a high C, most of the things that must be played up the neck I could move back down 5 frets and play on the C string.

Is this a reasonable thing to do? Or will it just be a different kind of bad on the C string?
Or should I just suck it up and sort myself out on the 4?

I think you should just practice more above the 12th fret and then it will come in time. A C string will not solve your problem. I don’t find it much harder to play in tune when I play high above the 12th fret. Even when I play my 6 string fretless I also play high on the C string till the 24th fret.

Having a lined board also helps for your intonation at the notes above the 15th fret. The sidedots on an unlined board can give you a distorted view when playing up high on the G string.

Also check if your bass is intonated correctly.