Planning a soil mix

I've got some young plants that will need repotting into 15 litre buckets soon and I'm working out what soil mix to make for them. Here's my idea:

30% local soil (clayous, reddish colour, mineral-rich soil from near a dried up river bed - Gran Canaria)
20% horse shit fertiliser (from a bag)
30% airy stuff (coco/perlite/straw)
20% worm humus/peat (from a bag)
Plus about a tablespoon of dolomite lime and a teaspoon of Epsom salts per 25litre bucket.

They will be outside and watered with collected rain water, mixed periodically with small amounts of Biobizz grow/flower/fishmix and molasses as I feel like it.

Does anyone have any thoughts or suggestions? I've always relied on Biobizz Allmix with added coco/perlite/humus/guano before but now find myself with access to plentiful and free local soil...just don't want to mess up by using too much of it basically. I did a runoff test and the Ph was about 7.

Mmm, I would skip the "local" soil (you just don't know whats in it!! bugs, mold spores, etc) and build (if you want to take the time) your own complete organic mix. I, personally, would not use coco for your home mix (unless you have used this before with good results).
You would be more like this.
33% Canadian peat
33% Worm castings
33% Perlite
Get this mix DAMP and,,,,
Then you'll need.
1 cup lime (per cubic foot)
4 cups rock dust
Now make a nutrient amendment mix, like this-
2 cups kelp meal
1 cup crab meal
1 cup Alfalfa meal
1 cup dry organic fertilizer ( Dr. Earth "all purpose" or "tomato and vegetable blend" and Down to Earth "all purpose blend" or "tomato tone".) Mix this together and add 2 cups (per cubic foot) of the mix to the above mixture.
You now have to lets this sit for 3-4 weeks (keep damp) and viola! you now have a soil that contains what you need to grow - NO synthetics need be used to grow!

Or maybe do this one:
https://www.rollitup.org/organics/539844-subs-supersoil.html

Don't want to bother with the work/time?
Premixed.
Dr Earth's POTting soil
Potters Gold
or anything fully amended and organic....

Hmm interesting, thanks. Might be tricky getting the crab/alfalfa meal on the island, I'll ask at the garden center..