This is not good advice to give to the OP, i.e. it's best to stay home and avoid the SSPX because there is no Resistance chapel near.
We are under a most serious obligation to attend the Holy Sacrifice (not the NOM - we are obligated to avoid the NOM). It is the third of the Ten Commandments and it is Precept of the Church. If one can make it to the Holy Sacrifice on Sundays and Holy Days then one must go, it is our duty and we are under a most serious obligation to attend - and we must go or commit a mortal sin. This is the teaching of the Church, this is the Third Commandment and Precept of the Church. The SSPX has valid priests and they offer the Holy Sacrifice that the Church obligated us to attend - if there is an SSPX Mass within a hundred miles or so and you can make it, then you MUST go.
In the Old Testament, sacrifice to God was one of God's most necessary requirements, and now the sacrificial Lamb is Our Lord, really and physically present under the appearance of bread and wine, it still remains one of God's most necessary requirements in the New Testament because of what it is - and it is for that reason that to miss it when it's readily available to us is a mortal sin.
There are many confused souls in this crisis who do not understand the seriousness of the obligation we are under to assist at the Holy Sacrifice every Sunday and Holy day of obligation. They confuse the NOM with the Holy Sacrifice and often end up at a NOM when they should stay home - but they must not confuse that with the SSPX's Holy Sacrifice, which we are obligated to attend if we can get to it under pain of mortal sin.
There are reasons to miss Mass, but missing it for the reason given above is most certainly not one of them.
That is your opinion. There are many other opinions, equally
or even more well-founded in Catholic Doctrine than yours.
100 miles? Who decided that? You sound like you're just making stuff up. You can't just pontificate from your armchair like some kind of armchair pope. In the Crisis in the Church, many things fall under "prudence" and "one's own discretion" with the primary goal of keeping the Faith and pleasing God.
I appreciate your love of the Holy Mass, but it is a fact that the SSPX has preyed on the beliefs of people like you ("sacraments at any cost") to control, frighten, and manipulate the Faithful.
Why do you think so many Trad Catholics sink so low as to go tattling to their SSPX priest, spy on fellow parishioners, or stay quiet when they shouldn't (even when staying quiet is sinful?) It's a fear that they won't be able to go to Mass if the SSPX priest gets on their bad side, or banishes them from the chapel.
An exaggerated doctrine, such as the one you put forth here, is the reason why so many SSPX Catholics are going along with the "changes in the SSPX" which are analogous to the "changes in the Church" after Vatican II.
After all, a softening up to Vatican II is nothing other than a liberalization. And that's not something to sneeze at, in terms of danger to the Faith.
If we are permitted to leave our OFFICIAL PARISH of the Catholic Church, headquartered in Rome, because of "danger to the Faith", then we can most certainly abstain from our local SSPX chapel because of "danger to the Faith". In both cases, one is permitted to abstain to preserve one's Faith, regardless of the presence/absence of other Mass options.
If you want to talk about Trads committing mortal sin by missing Mass on Sunday (after 1970), then the "line in the sand" would have to be missing Mass at your parish church -- not your SSPX chapel. Once you leave the official Church structure and go into the realm of survival, lifeboats, and epikeia, then there's a lot more grey area than you care to admit.
As a Trad, I believe that we have an obligation to keep the Faith, and so we are obligated to abstain from the Novus Ordo and any other Mass which places our Faith in danger.