I would say, "Yes," although, I agree that only Jesus Christ Himself or the Church, at a later date after his ceding the Throne of Peter (either through death or abdication), could pronounce him as being a formal heretic. Until then, traditional Catholics have not only the right but the duty and obligation to resist him and his many errors and those of his predecessors.
Jehanne do you really think that a heretic is going to willfully abdicate? Did that ever happen in the history of all the anti-Popes we have ever had. In fact it was precisely this point that made St. Vincent Ferrer declare in front of Benedict XIII, all of his Cardinals, several Kings, Bishop's a schismatic etc... After St. Vincent made that SV'ist declaration (once again the realization of something that was true already, so that the maxim that the Apostolic See is judged by no one is kept fully intact) all but a few Cardinals remained loyal to Benedict XIII so great was his moral authority that Historians rightly call Martin V, "Brother Vincent's Pope." Heretics can be judged by the Church, a true Pope can only be judged by God.
The Church cannot declare a true Roman Pontiff a formal heretic either when he is alive or dead. It would be only because the man was already a heretic that he could be judged by the Church, so now we are back to square one. Instead of waiting for hundreds of years for something that honestly I highly doubt will ever happen, just realize the spiritual reality of what you are saying. If you say that in the future such a declaration should happen, then that would imply necessarily that our position was the true one all along. That is that they never were true popes...
I agree you have a duty not to accept heresy, maybe not resist a true Pope. Nevertheless you are trying to preserve the Catholic faith whole and undefiled. So I am happy that you are resisting a false claimant, although you might think that he is a true Pope he is not. So in the end by resisting all their errors you will have kept in tact the fullness and integrity of the Catholic faith.