You seem pretty well informed about how things work. Many people forget about the risk of price decreases. And not to mention downvotes.
Did you think about powering down your SP? To have a bigger amount available for buying upvotes?
It might increase your growth, but if course it is a small risk.
And did you calculate how big the price decrease of Steem should be before a 40% profit turns into loss?
I am trying to avoid powering down but seen others use that strategy and it seems to be working. Yeah, there is a fine line between abusing and not abusing bots. Bot abuse / use can cause downvotes so there is that to consider also. I am not saying I use the strategy lined out, just that its possible.
I never did any exact calculations on how price fluctuations would effect the ROI. Crypto is so unpredictable Its almost pointless to do so. If the price takes a big swing do the calculations then and if something is no longer profitable adapt an change.