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I would say the first step is to continue insisting on having a meeting with the bishop, with representatives of this group, and to say that, since we are living in a synodal Church, as everyone says, please listen to us in a synodal way and give us the justice we deserve. We are a large group. We only want to worship the Lord as our forefathers did, as the saints did. We do nothing else. We accept the Pope and the bishop, and we do not engage in polemics against the Council or the Pope. When they say this, the bishop should grant these requests, I think.
The first step would be to continue asking. If the bishop, despite this, categorically refuses, then they could ask Rome to intervene. They could write to Rome and ask the Holy Father or the competent congregation to instruct the bishop to grant this request. If this is not done, then they could consider inviting a priest. This would not be legal, because the bishop would not allow it, and the Mass would be illicit. The priest could be punished by the bishop, for example, with suspension, and this would not be good for the priest.
I think the most practical way, in this extreme case, when even Rome does not allow it and the bishop continues to resist, would be to ask the Society of Pius X to send a priest. In this case, they are not subject to the bishop, and they have a kind of semi-juridical status, not yet full, but in some cases, they have faculties, at least the faculties to hear confessions, which are valid. By contrast, diocesan priests whom they might invite, and whom the bishop would forbid, would not have the faculty to hear confessions. This is different from the priests of the Society of Pius X, who have ordinary faculties applicable everywhere.
This could be a last solution, I think, but only after trying the steps I mentioned.