I took a calendar and laid out every possible test date for SAT, ACT, and subject testing starting in January and going through the fall to try to figure out when my kids could test. Then overlaid it with school breaks, key EC event like state tournaments, recitals, etc. to try to create a plan for testing. It was useful to look at it in the fall of junior year to try to figure out the least stressful calendar (or at least to know in advance where the pain points were going to be).
I would not ignore subject tests until the last minute if your D thinks she want to apply to any schools that “recommend” them. But have her take ones that play to her natural strengths, whatever those are.
We did at home SAT prep. We found it useful to set aside a couple hours every weekend starting fall of junior year (usually it was 2 hours late Sunday afternoon). After initial review of the official prep book materials, they took one test to see where they stood. But we didn’t do it all at once, just the sections that would fit into the allocated time. Then scored to determine overall standing. Both needed more math prep than reading/writing, so most sessions after that focused on math. Kid takes a practice section, scores it, we review the ones they missed together. They made flashcards to review going forward (one said, “Don’t be a stupid rat!” and had a tick mark every time the mistake was just a dumb math error – there were a lot of tick marks. 
). They reviewed the cards before every practice test and the real test. Honestly, we could never find any time during the week or more than a couple hours on the weekend for this, but slow and steady paid off for them over the course of the year. Neither reviewed for the ACT more than just to look over the format and formulas. Both took it, and scored pretty equivalent to their SAT scores (I think the prep carried over between them to an extent).