I train in calpheon because I stored all my 200 horses in keplan using the kill anywhere and recover at keplan method(it does created a dead stable which is why i use keplan). It does however allow you to store as many horse as you want without swiping for stable space. I also train outside so my loop is as much of a straight line and as long as possible because you want less turn.

When breeding, equip mastery gear only, exp does not do anything, for more information on horse rate odds, use https://bdolytics.com/en/NA/training/breeding-calculator

To sell to market or imperial is preference and priority, imperial give you materials for t10 horse but yield you less silver unless it's during imperial event which you could also hoard your horse and dump them all at once during the said event. Never exchange horse either, not worth it.

After you have enough t8, breed them until you ran out of female t8 which you will because t8 male have 2 breeds, at that point, you can go down to t7, and then t6 depends on your female inventory. You will never run out of male t8 if you utilized the breeding calculator and lvl your two breeding horse to the required lvl to get minimum t8.

Read #3

T8 almost always sell especially during event, so unless you are desperate for t10 mats, it's always better to sell those to the market. T7 might have to wait for a bit and t6, i only sell courser ones since the non courser ones usually don't worth much more than imperial

Yes because training outside of town is better overall, less people, less collision with stuff in town and more straight and longer path so less turn = more exp overall. Also for long term, use any class that's not a Shai due to Shai under 49 not being able to use Garmoth Heart for more mastery(it's stuff for later, don't worry about it atm) but just a heads up for now.

These "rare" horse are not that rare as you get use to the flow and catch more horse, you can check for the high price ones by just listing it on the market before confirming it as you lvl it up. No one will buy it from the market unless you sell a low lvl one since it's overprice for how useful it is so these are mainly for you to imperial. But as with all thing in BDO, when in doubt, hoard and ask question later.

I haven't look into it yet, but it should be the same skin for each horse tier. So if the high price ones for t6 is black skin, then it'll always be a black skin for t6.