It seems to me adding to an SEP IRA would be ill-advised because, if I've got this right, would reduce this year's income value on which you must pay taxes. But you don't need to reduce this year's income.
The principle many people use is to fill-up the lower tax brackets in low income years under the presumption that after they retire, they'll have untaxed funds that will have to get pulled out (because of the required minimum distributions regulations).
I think what you want this year is to convert SEP money to Roth, and fill up those low brackets. You can buy tax software now, put in estimates for everything, then keep jacking up the conversion
and stop when the marginal tax gets to where you want it.
And if you have money you want to save for retirement, put it directly into the Roth.