I'm not quite clear on what you're asking. I've got the part where you have to take your first RMD by April 1 (
note 1, not 15) of next year, and you've taken half so far (in the form of QCDs).
It sounds like you want to use the other half for income. But it's not at all clear whether your concern is over CG distributions inside the IRA or outside. In what follows, I'm assuming that they're inside, though the next couple of paragraphs apply either way.
I think that CG distributions are a distraction. IMHO they are simply return of principal. So it doesn't matter if you sell fund shares in your IRA to generate cash or take CG divs, it amounts to the same thing.
The reason why I view them as return of principal is: a fund owns various securities. Say that some of them appreciate. The fund has unrealized gain. If the fund swaps those securities for other securities, the principal is unchanged, but the fund has realized gain.
No difference in your principal. But due to tax laws, in December, the fund is (conceptually) required to sell off some of its principal to pay those cap gains divs. (For investors who "reinvest" those divs, it doesn't have to sell some holdings; on paper the investors get cash that goes right back into the fund to "rebuy" the same fund holdings.)
If you're tapping principal for income (as I feel one does by taking CG divs in cash), then the question becomes what is a good time to sell principal? I leave it to each individual to guess whether this is a good time to lighten up on investments or a good time to be buying.
The other half of your question seems to be, given that you're going to be taking a certain amount of cash out of your IRA for income in the next 12 months and four days, whether you're better off taking part of that now, or all of it next year.
If you're in the 24% bracket in both 2018 and 2019, then from an income tax perspective, it won't matter. Either way, that 2nd half of your RMD will be taxed 24%. But if it would push you into the 32% bracket next year, you'd be better off taking it now - even if you leave it invested in funds in a taxable account (so as not to be tapping principal now).
There may be another concern. In the 24% bracket, you're likely paying IRMAA on Medicare. (Some retired people have medical coverage paid for as part of their retirement benefits; they may be protected from IRMAA.)
I haven't gone back to cross check the IRMAA brackets against the income brackets, that's something you would have to do yourself. Especially since IRMAA is based on MAGI that includes the 1
5% of SS that isn't otherwise taxed and non-taxable income. (If your CG divs are outside of your IRA, they would also get included in MAGI.)