While it is clear that we should avoid the Novus Ordo Mass if there is no TLM available I am wondering does the same apply to the Indult Masses? If no Resistance or SSPX Mass is available is it better to avoid an Indult Mass from the ICKSP or FSSP and spend the Lord's Day in prayer and reading Scripture?
Yes.
As Lefebvre said, when you go to that Mass, you don’t just attend that Mass, but an entire milieu:
There’s the advice in the confessional; the sermon; the conversations over coffee after Mass; the new fasting discipline; biritual clergy (sometimes); priests and faithful defending the orthodoxy of the errors of the Council and post-conciliarists reforms.
Eventually, some of those germs penetrate your immune system, and the conversion process begins.
Some might opine that even a willingness to attend an indult is itself already a sign of conciliar infection.
Mr. Salza is a good example of this (he admits to never having completely left conciliar indult Masses, and today, he defends V2 and opposes Lefebvre almost across the board).
Going beyond the Mass, in the wake of Francis’s Traditionis Custodes, it seems the questionable forms of the new sacraments will be used by these same groups (which might include invalid last rites performed with vegetable oil), etc.
All of that bound up together might present or introduce obstacles to persevering in the Faith.