What soil to mix and what to use ?

Apart from the worm castings, I would recommend not using any of those products!

Honestly I wouldn't add anything at the seedling stage. More often than not, people add too many things moreso than not enough and seedlings really don't need anything more than patience that first week or two.

They discontinued Ucann lineup aka the Herb thrive. where did you find it? The living soil from them should be fine on its own for the first few weeks. after that you will want to transplant and amend the new soil in the new pot. It works well to mix the 444 and 284 in a 50.50 ratio as well. then add in some seaweed into the water to boost the K a bit. Use worm castings with caution as it can muck up the medium fast is used in to high of a concentration.

eliminate the guesswork and go soilless :P

For seedlings and vegetative state, I use Gaia Green all purpose 45ml/gal when mixing in soil, then top dress 15ml/gal every 3 weeks until you switch to flowering. Then top dress with Power Bloom 15/gal when in flowering. Last top dress should be at week 6 of flowering if you're harvesting around week 8-10. For the soil I would mix 75% living soil 25% worm castings and 45ml/gal of the 4-4-4. Hope this helps! Happy growing!

Sorry if that sounds harsh but that's the best advice I can give you pal...

Not to be funny but what do you expect the plants to grow them selves? Try and read something first before asking to be told what to do... Come on learn something. Hope this helps..