90 degree corners no problem. Slotted fence posts with T&G panels.

How to deal with odd angles? How to get/improvise fence posts?

Have a look at fence cant rails to use as posts.

Maybe not the best fencing option for the job at hand. Or change the shape of the fence line so you close to 90 degree angles.

 

Or use round posts at the corners and just notch a flat at any angle you want.

featheredge it, angles are easy to accomodate.

One option is to notch the posts so the rails go into the post. Taper the rails so they don't have to be a precise angle. Probably rots quicker...

 

https://www.hooverfence.com/how-to-install-fence-panels-at-odd-angles

 

Or perhaps bend your own arris rail brackets. Commercial ones are typically 90 degree..

 

Reporting back.

 

Used galvanised fence clips. Easy to angle any which way. Time will tell how many need replacing how quickly.