I’m a Randonneur / Audax rider and my season started last week. I’ve pretty much got a 200km or longer ride scheduled every weekend from now till September. So I understand your dilemma.
Just follow a full plan at low volume. Start with SSB1, SSB2, any build, and then pick Century specialty. Put Tues/Thur as your mid-week hard days and Sat as your weekend hard day. What you’ll be doing is swapping out the Sat ride for a long endurance ride every weekend which is something the plans definitely mention.
You can always add a tempo or sweet spot ride to your schedule on Wed. The one thing I find is that quite frequently I can’t do the Tuesday hard ride if the weekend was a particularly long event. That is 100% okay and rest is your friend. When I do a 400K, I definitely need a full week of rest and longer rides than that require even more recovery time.
The TR plans are meant to be done in the off season for strength growth. But it’s definitely worth doing some VO2 and threshold work during the long ultra season to keep your high end high. (IMHO)