Much will depend on what type of Alstroemeria you are growing. Some are deciduous and a few are evergreen. The deciduous ones will naturally go though a growth decline after blooming and the yellowing or dried leaves is just an indication they are done for the season.

You don't need cactus soil - they are easily grown in any free draining potting soil. And it is always best to use a liquid or fully water soluble fertilizer for containers frequently and at dilute (1/4-1/2) strength - the spikes are just a waste of money. And when a flower spike is done blooming, pull from the base to remove. Do not cut.

btw, in your climate (and mine) these can be grown very easily in the ground, although some will tend to spread rather aggressively.

Really? I thought growing season wasn't until the end of September in SoCal.

And I read somewhere that I shouldn't expect flowers until year 2 if growing from seed. Is that a good general rule of thumb?

For many varieties, the end of the growing season is when they stop flowering :-) btw, growing in containers can put stresses on plants that they do not encounter in the ground....and these can artificially shorten the growing season.

And I have never bothered growing from seed.....intentionally.

You wouldn't happen to have any tips on how to care for them while they're dormant, would you? Like, should I continue to water them, just less frequently?