I'm obviously a bit biased, but...
Torval and Mahon are the two Trade Powers. Both offer Trade Bonds and increased Rare Commodity profits. In my opinion, Agricultural goods do not make very good profits, while (noting your aversion to Indentured Servitude) Imperial Slaves are arguably THE most profitable commodity available outside of Rares. Also, as noted above, the most profitable trade routes are often based out of Torval space. Of course, these are passive bonuses available to all whether they pledge or not. Pledging allows us to keep Torval in Power, therefor allowing all of our galactic community to reap these benefits. Which again are some of the most lucrative trading routes you can find.
Also, consider the player base. Mahon has a very large player base, which helps keep them at the top of the Rankings. And they are a well-oiled machine. Torval has a very small, one of the smallest even, player bases. Which I happen to enjoy, as we are a tight knit community where each member can have a say in the weekly strategies if they choose to do so. And we have kept Torval just as competitive as the more populous Powers.
Check out the recent Recruitment Memo from Parratich:
https://forums.frontier.co.uk/showthread.php?t=229248
Also, check both sub-reddit links in the stickies above for Torval and Mahon. If you choose to go with Torval, there is a thread there from JemyM inviting you to join the Strategy discussions.
P.S. As @Apos pointed out, Mining is counterproductive if you are selling in Torval systems due to the mentioned discount. However, as a Trader, this allows you to purchase cheaper Mined Materials for possible trade routes, which are the second most profitable commodities after Imperial Slaves.
But like I said, I'm a little biased!