You should not keep attending SSPX Masses because you are in schism with the man you believe is the Vicar of Jesus Christ.
If you come at it from the obedience perspective, New Rome simply dislikes attendance at SSPX chapels. She doesn't actually condemn it, and in fact has said that Catholics can attend there. While I get that that *in and of itself* is not sufficient reason to prefer an SSPX chapel over an Indult, its not schism. Rome doesn't even consider it to be schism.
On the other hand, if you come at it from the Sedevacantist perspective, you have to assume una cuм is actually some kind of sacrilege or schismatic act rather than merely an error of fact, which seems hard to substantiate.
I think what you really take issue with (logically anyway) is moreso the R and R *position* than the specific act of attending a mass offered by SSPX priests. But the problem is "R and R" is more than one position, and there are different nuances to each.