AusBonsai

Sandy loam is a soil type and should not be used in a potting mix. Fine for a garden bed.

The nursery industry moved away from mixes containing soils some decades ago. Modern potting mixes should not be called soils as they are not. They are a potting medium/media.

The problem with mixing aggregates like scoria and pumice say into a premium potting mix is they no longer function as well as they should by supplying the air filled porosity stated on the packets. However, adding pumice would be better than scoria. It is lighter and will still allow more air in the mix. Another alternative would be Perlite. Generally cheaper and lighter than either option. A good produce store should have either pumice or pelite.

I wouldn't be too keen on coco coir for pines. may hold too much water.

Another thing to consider is not potting into too large a pot and just keep the water and fert up. You will most likely get the same growth you just may have to repot more often. Which also gives you the chance to correct root issues.

Large pots have been touted as being a go to to get good growth when the ground isn't available. The nursery industry which grows plants quick and are after the best growth possible will never go from a small pot to a very large one in one go. Rather they go up one size at a time. sometimes multiple times a year. This ensure you don't have a large pot with few roots which stays wet too long, rots any organic components and lead to poor root health.

Thanks for sharing and some really good pointers here along with others shared also.

I'll go and get what products I can and will keep in mind the drainage, aeration and some organics for the mix.

I definitely want to get this right as being a hopeless romantic, i got a few more pines over the weekend including a mugo

But I think I cant do much with them until end winter early spring? Cant wait that long

If you can find a good commercial potting mix made exclusively from pine bark it will work well.
Unfortunately that is becoming almost impossible to find these days unless you can buy in bulk from a good supplier.
The stuff you get in bags these days is a very inferior product and the so called Australian Standard (the ticks on the bag) cannot be relied on any more. A while back I bought a bag of supposedly ''great quality'' mix and found it was made up mostly of sawdust and had a pH of 8.5. Garbage in other words. The good Deco style mixes are long-gone. Quality mixes are now reserved for nurseries it seems. The bagged stuff can be improved by sifting out the fines and adding gravel and scoria. About 50-50 organic/mineral for pines. If it's still too fine add orchid seedling bark until you're happy with drainage speed.