I'm not sure if I understand your concerns.
I will expand mining with loan. I will repay with BTC.
So, you will benefit if BTC revalue.
Would you prefer a fixed USD repayment, a mix ?
What you are proposing is essentially a one year business bond. The problem is that it's unsecured and includes currency risks. 3% isn't enough interest. However, I also understand the concern for yourself, because if the difficulty shoots to the moon, you might not be able to pay back the installments. But I think that I have a solution.
Start by bumping up the APR to something more in line with a high risk bond, say 12%; but include an escape clause. For example, an agreement to the monthly payment, but if the difficulty is set to a number higher than a certain limit, all future payments are waived. This protects you from a spike in the difficulty, while also permitting the investor (myself) assurances of a reasonable profit margin. It works because there is a strong corollary between the difficulty and the market price trendline. So if the limit is breached in six months, I only receive half of the bitcoins but it is a fair assumption that the trade value of that half of bitcoins had significantly increased during those six months. At this time, I'd be willing to set the high difficulty limit to 250K, roughly triple the difficulty at this point. Run your math, if you are interested, to judge your risks. On risk to me is that it's possible for the bitcoin price to decouple from the difficulty, but I would say that the odds of that occurring within any one year period are remote.
I understand now, and your logic is flawless. If difficulty goes too high, it will be hard to repay that 3% mo. in BTC.
Anyway, at the same time as minimizing risks, I wish the investor may benefit also from a BTC value rally.
To make things simpler, I'm thinking on offering a mixed repayment type : capital will be repaid in USD and interest in BTC.
e.g. : for every USD 1000.- you invest, you will be paid 12 x USD 83.33.- and 12 x 40 BTC.
What do you think ?