I think the general wisdom is that you can hold a peak for, at most, about a month. If you want to ride a peak through the season, you may not want to finish specialty at the start of a season. Having said that, you probably have a few options.
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Finish SSB, do General Build and then reevaluate. What are your weaknesses/limiters? If you find VO2 is holding you back, put in a 2-3 week tuneup focusing on VO2. (Good guidance can be found here: Progression workouts - #7 by bbarrera) If it’s sustained power, tune that up (perhaps borrow a week or two from sustained power build or a later SSB II block). Then do specialty phase from there, which would likely have you peaking a few weeks into the season, which you can then carry through for a month.
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After build, move back to redo SSBII at your new FTP. SSB II has good VO2 work to keep you sharp there and will expand your base further at a higher FTP. Then move on to specialty as the season begins (maybe do LV to account for TSS/fatigue from races). Put in tapers as needed to prep for A/B events. If you have a key A event, you can chop/drop parts of SSBII to carry you into those events at a specialty peak (end of week 4 and 8).
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Give the new plan builder a go. I haven’t looked at it much, but it seems to me a pretty great solution. Add in your events you’re targeting and see what the @chad brain builds to give you the best fitness for those events.
As for what a taper should look like, you want to keep the intensity but drop the volume way down. Basically the 30-45 minute versions of the workouts you’re doing during the phase. For XC, it’s probably a focus on bursts and VO2 efforts.
To avoid burnout, you may want to put in a week of “nothing” here and there. I like putting in a week of only workouts I enjoy, or cut everything way back with the minus versions, and/or go out for a ride with the only goal to have fun or try out a new route (knowing I’ll go slow, get lost, grab a coffee/beer whenever I feel like, etc). Mentally it’s a reset and I come out of them ready to get out and smash the next few weeks. I try and put them in about every six weeks (between SSBI and SSBII this year I went bikepacking for a long weekend and came back wanting to get back to it.)
Sorry for the tome, but hope it helps.