You have bought your plane ticket already, I guess.
the airports of Santiago, Vigo, Porto, Madrid avoid long travel or hassle.
redexpressos has one direct bus leaving Santiago at 12:00 and arriving Lisbon 20:30h (50€, 9h30 travel). If you get in at Vigo it is still 8h to Lisbon (34€).
One of the best connections to Lisbon I see in the combination of the 4 hours bus transfer SDC to Porto and additional 3 hours in another bus oder train to Lisbon. At least I can bear 4h pressed in a "can", but 6 or 9h would be a challenge.
To spend less money more changes seem to help. SdC - Vigo is 6,70€ (every hour), train Vigo - Porto ~15€ (bus ??), while SDC - Porto with ALSA/redeexpressos is ~30€. The (often direct) bus from Porto to Lisbon had been 19€. As busses from Valenca are about the same cost (22€) and more frequent, going from Vigo to Tuy by ATSA and walking over to Valenca is better for the price, but time consuming.
Vigo (bus station) - Porto has up to 3 buses per day @17€. You arrive at Campo 24 Agosto where the bus to Fatima/Lisbon is leaving.
Be warned! Vigo is a bit complicated if you need to change. The train station is on the water front, while the bus station is somethere up on the hills. To my experience ATSA does not stop at the bus station. Porto