Wow great questions! I'll do my best to answer them. Firstly, I kind of did a combination of both regarding rentals, I rented at the lowest prices I could get ahold of for the cp and also rented a couple of gold cards but not the expensive legendary as they were renting for about $15 a day by the time I was looking near season end.

Something I've noticed is that the peakmonsters charts for cp per dec seems to not catch the occasional super low rental prices that I see directly on the splinterlands marketplace, for example I catch a lot of rentals for like 3-7 dec per bcx a day for legendary cards where all the other available ones are 50-200 dec a day, like someone was just really wanting to get that card rented so they way under valued it, but they get picked up so fast that idk if they even make it to the peakmonsters site or other sites. I often get the "no longer available" popup, but it was worth the sluething around for me to get such massive discounted rental rates, beating the best cp per bcx prices i saw on peakmonsters. I hope that makes sense.

Secondly, YES someone needs to chart that!! I've never gone beyond silver 1 except for very momentarily into Gold but not enough to report on what the dec reward really was, unfortunately.

Yes I believe around 30-35 dec was the best I got without bonuses, but I generally am always using several bonuses which does make a big difference when starting from as high as 30, 50% or better bonus from gold/alpha etc can bring it up to 45-50 dec.
So in my mind, that's significantly better than the 12 dec I get in bronze/lower silver WITH bonuses, my bonuses don't make as much of a difference when starting from like 8-10 dec - where 50% bonus only adds about 4 dec.

That's something that makes renting several different legendary or alpha or gold cards a better ROI compared to just renting a ton of gold harvesters - since that doesn't improve your deck of cards, I rented the cards that help me win more AND that give bonuses to improve my dec reward at the same time, and tried to snag them at super low rental prices (almost seems like the person made a mistake they were so low!).

That way I made sure I was actually paying off my rentals through the earned battle DEC rewards each day (So I ended the season with about the same DEC balance as I started with, but got all the reward cards from all the chests as my profit). So I'm not sure what that ROI is, I got a lot of cards from my daily quests in addition to the season reward cards.

Right now you can get way better deals on rentals than near season end, so I would recommend looking around for those super steal accident-looking prices on rentals (they're like fluke prices where only 1 card is available at anywhere near that low so they get picked up super fast) and rent them for the remainder of the season since all the prices will go soooo much up near end of season (just a noob piece of advice for those like me who didn't realize the rental prices would go up that massively near end of season), and pay off those rentals each day through battle wins so that the cp is essentialy no cost to you.