The OP on this thread was asking about the difference between the What’s My FTP app for Garmin Connect IQ versus the TP calculation on XO. Here’s the explanation for that –
Difference ‘What’s my ftp?’ and calculated ftp Xert. - Support - Xert Community Forum (xertonline.com)
As to your TP decline first know that TP in Xert isn’t necessarily FTP as calculated by taking a traditional FTP test (RAMP, 20 min or 2x8).
Did you do a traditional FTP test or a BT (breakthrough) workout from the library?
BT workouts are much easier than a RAMP test but they will challenge you when done correctly (takes some practice).
Xert is looking for how well you perform especially when fatigued and measures several points along your power curve. The TP calculation is relational and factors in your PP (Peak Power) and HIE (High Intensity Energy).
TP is also susceptible to decay based on a setting which is normally set to Optimal. See Account Settings, Profile, and click on the ? icon for Decay Method for the explanation.
No activity for days plus little activity consisting of BT events will accelerate that decay.
Your approach is certainly viable if that is what you prefer to do (HIITx2 + low intensity rides indoors or out, Zwift, etc.).
You can use the analytics in Xert to monitor your fitness and set the TED (target event date) in the past. Then recommended workouts will include a variety within your selected focus duration (Athlete type) without phased progression. You can also select a different athlete type using the Filter option for even more variety.
You can manually set your signature values if you feel the numbers are off currently and you are not up to the task of more testing. Plus you can enable No Decay during your float period. Remember to turn it back to Optimal when you can challenge yourself more often.
There’s lot of different ways to use Xert and the flexibility takes some getting used to coming from select-a-plan platforms.
I am recreational rider as well but I like to get fit by mid-summer to handle long rides at a reasonable pace (for me) and shorter rides that may include some hammering, a personal KOM on occasion (for kicks), or hanging in with a passing A/B group for a few minutes. 
Regarding targeted improvements you can select Ranking and view your spider chart to see where you are weakest. Mine looks like a starfish with limbs lost to the sea especially the northwest quadrant. While I could select a type such as TT to work on that weakness I’m just not interested. I’d rather stick with what I am better at than struggle with what I’m not. Cyclists who are competing would want to work on weaknesses to round out their charts. YMMV