This is a tricky spot to be in, but if I were in this situation I would skip sweet spot because I don’t think it will get you what you need in such a short amount of time and with the demands that CX asks of you.
I would focus more on 2 high-intensity workouts per week, one threshold workout and a lot of bike handling work. The high intensity sessions will prepare you for actual race scenarios and getting some bike work in will hopefully offset your lack of fitness. I also think you can make up for a lack of bike fitness with good run preparation. Running is what really gasses a lot of strong riders.
To me this is less about trying to find some form than it is just getting the body ready for really hard racing. A week for me would look like this.
Monday – Easy Ride
Tuesday – Anaerobic workout. 30/30’s aiming for at least 15 min of total work. Example, 2 sets of 15x30s with 30s of rest between intervals. Full gas, ignore trying to hit a power target, just go as hard as you can for 30s.
Wednesday – Easy Ride w/a focus on CX-specific drills. Figure-8 cornering drills, standing starts, a little bit of running, barrier practice
Thursday – Threshold. 2x4min @ full gas 2 times. Or shoot for 105-110% of FTP or at least 90% of heart rate max
Friday – Rest
Saturday – 90min endurance ride on trainer or 120 min outdoor
Sunday – Free Day
Once the season is underway I would do one high intensity session on Tuesday or Wednesday and let your weekend race be the other. I’d fill in the other days with more bike work, running, endurance, and sweet spot riding. I would start to incorporate sweet spot now because I think a thursday threshold workout will be too much going into the weekend. So my in race week would look something like this…
Monday – Off or 30 min on trainer easy
Tuesday – 30/30s or 60/30s (15-20 min total work)
Wednesday – Sweet Spot (3x10, 3x12, 3x15, whatever your body can handle)
Thursday – Easy
Friday (assuming you race Saturday) – 30 to 45 min opener workout (3x60s full gas, 2 min threshold, 5 min sweet spot, the rest easy)
Saturday – Race
Note: if you race Sunday than you do your openers on Saturday and you have Friday to do as you please, light jog perhaps?