This is a very bad idea for a number of reasons.
If I had a Bitcoin for every time someone's told me that my investment ideas were bad... :-)
First this is a very expensive way to invest in your retirement. To pay $1,500 in order to "open (setup) your account" to be able to invest in BTC is a lot of money especially when considering the limited amount of funds you can invest in retirement accounts.
The amount that can accumulate in various retirement accounts, then get converted to a Roth IRA, is not that small. It adds up and grows over the decades. If you merely put $4,000 away each year for 10 years, that's $40,000 right there alone.
You are right that paying $1,500 to set up your account would be ridiculous if you only had a few thousand to invest, but it's not so ridiculous if you're investing $15,000 or more.
The question you have to ask yourself is, do I expect to save more in capital gains taxes than it costs to set up the account? That depends on the amount you're investing, what you expect the price to do, your income bracket, etc.
Second Bitcoin is a very risky investment for retirement. Bitcoin is likely a good long term investment, however it is very speculative and carries too much risk for retirement. There is a good chance that investing in Bitcoin could make you very rich, but there is also a good chance that Bitcoin will become close to worthless.
It's OK to have some risky investments as part of a retirement portfolio, but I would not recommend that anyone put more than 10% of their net worth into bitcoins at the very most.
Third you have the issue with potential self dealing. If your IRA LLC were to hold BTC via the traditional way of a wallet holding/being in control of private keys then any time someone sends you a random "donation" to your public address you could potentially be accused of self dealing. Many very large BTC addresses often have small amounts of BTC with spam attached to them. The only real way to avoid this is to own BTC via a large exchange or other central fund. This would create risk that this entity could fail and/or steal your coins.
I don't really see how receiving unsolicited spam bitcoins from absolute strangers violates the self-dealing rule. Could you elaborate on this one a bit?