"return of capital", before the end of the year, includes call premiums and capital gains/losses. At he end of the year that gets reconciled on your 1099. Where there are call premiums that carry over the year-end, they will end up being "return of capital", the remaining distribution will be relegated to dividends and long or short capital gains. The call premiums associated with settled contracts will be tagged as long or short capital gains as the case may be.

"return of capital" that stays that way on your 1099 really amounts to tax-deferred income; either until you sell the ETF or on next year's 1099.