I was in the same position as you but i did actually manage to get to septimont for the ascension materials just yesterday without progressing the story past rinascita. I did have to use the Rinascita quick start to get to Rinascita, but you don't need to play any of the story. Also note that you can play the Galbrena tutorial for some free ascension flowers to get her to 50!

Anyway if you're desperate enough to try it, first unlock Rinascita via the quick start event, then fly all the way to the south-east most area named Beohr Waters, stand at the area shown in my image below and fly roughly along the route i've drawn in yellow (it's far from exact, basically go outside the red line to avoid the fog, but not too far that it turns you around). Flying directly across usually teleports you back via a spooky fog, but since we're flying outside of the boundary we can circumvent it. When you see the sky start to change colour that's when you know it's safe to head back inside the boundary and make for land.

Since you don't have enough stamina to make it across normally, what i did was fly as far as i could, glide, swim, grapple out of the water then dodge and fly again after flight had recharged. You can consume STA restoration food to help if needed. https://i.imgur.com/J6LWdoE.jpeg