The short answer is that you can potentially save a good bit of money by snagging Promo tickets for trips on fast trains like the AVEs before they are all gone. How long the cheap Promo tickets are available depends on how soon other passengers grab them. You might benefit by reading the background information on Spanish trains on the Seat61.com website.
The cheapest ticket source will be renfe.com. Sources frequently recommended in the past (trainline.eu and loco2.com) have recently started assessing service charges, though I believe they may be relatively modest. You should at least price out your tickets on Renfe so you know what the basic cost is. You can then judge whether the trainline or loco2 surcharge is reasonable given the convenience those websites offer. By buying Promo tickets early, you will save far more money than you will lose in service fees. You can see the fare variation in action on the Renfe website. Price a few of your tickets for travel tomorrow, then for February. You'll note that there is some variation from one departure time to another. Always pay attention to the duration of the trip, because there are sometimes much slower trains running along the same route. They are likely to be a lot cheaper but may take 2.5 or 3 times as long.
Renfe.com is sometimes tricky to use, being prone to switching back to Spanish midway through an English-language transaction. Not impossible (I managed to buy a ticket that way last spring), but requiring patience. There have been references to detailed online instructions posted on this forum in the past. What I've found by Googling is 11 years old, and I'd be reluctant to count on its continued accuracy. Perhaps someone else here knows where we can find more up-to-date step-by-step instructions.
One trip you do not need to worry about in advance is Madrid-Toledo-Madrid. That fare doesn't change, so you can wait to buy the ticket in Spain. Note, though, that there can be significant lines at Madrid's Atocha Station, so you should plan to use a vending machine. I think Toledo tickets may be sold at a different counter from the one dealing in long-distance tickets, where waits of many hours are to be expected, but still I get twitchy at the thought of buying a ticket from a human being at Atocha Station. That is an experience to be avoided unless you are looking for a way to raise your blood pressure.