Daily suggested bike threshold workouts too hard

18 mins at 105% of FTP is gonna be pretty much a maximal effort. 2 of them would be awful. Not surprised you had to quit.

A few thoughts:

1. Did you "Accept" the new FTP at the end of the test? If not you can manually edit your FTP in Garmin Connect.

2. Are you using the same smart trainer/power meter to do your DSW rides as the ramp test?

3. How is your cooling fan situation? 18 mins at threshold will generate a LOT of body heat, that you might not see in a shorter guided FTP test.

Suggesting a an 18 minute workout @ 105% of your FTP seems flawed to me, as you can infer your FTP quite well from a 20 minute test, so it's a little surprising it would suggest >100% for 18 minutes.

I think you need to take the workout suggestions with large grains of salt - I often run 2 watches in parallel, and even recording the same activities using the same power or HR monitors will elicit completely different daily suggestions. 

I find they tend to under-estimate capability for base, and over-estimate on the 'all out' efforts - I even had my watch suggest a 45 minute run, or which I was apparently supposed to be cracking out 3 sets of 4:30 /mi pace running for 3 minutes - needless to say I did not execute that workout!

Yes I accepted the new FTP at the end of the test and it shows up correctly in both the Performance widget and Garmin Connect. I'm using the same Tacx Neo 2T trainer to do DSW rides as the ramp test. There's a fan in front of the trainer and I wasn't hot.

the ramp test usually will give you higher estimated FTP value than your valid one (it depends on many factors of course but in average, it gives always higher value). the most accurate FTP test is the all-out 60min :), but the "5min all-out+10min rest+20min all-out" short and well-know version is also good. If you do a ramp test, i suggest to use 0.70-0.72 multiplier from the best 1min (and not 0.75), it will be closer to your valid FTP. So i think now You have a 230-235w FTP right now. but if You have a link for your ramp test , i can check it from the HR too

Thanks that makes sense. Here's a link to my last ramp test.

https://connect.garmin.com/modern/activity/8005463855

I just checked today's daily suggested workout and after I failed to execute yesterday's 260W Threshold workout it tried to give me an even harder 265W Threshold workout today.

ah, it was not the "classic" 25w/min step ramp, it was a 12mins (3x4mins) under/over. i suppose it is using a few "gimmicks" from your threshold hr/power ratio and your max hr setting value (if that is set too high than the power estimation could be too high imho, probably it is the reason why it gives you higher estimated FTP than the real one)

if you can create a workout like this, it would be able to give you better estimation than this Garmin inbuilt one :

start from 100w, 1min, and increase it with 25w / min (or if you have time, create it with 5w/12sec steps, that will be much smoother), so 100, 125,150,175,200,225,250, create it till 400w (that will be enough now :) ) , you will fail somewhere at the 350w step i think, and your FTP will be the best 1min avgpower x 0.7-0.72 .

here is a short description about this kind of ramp test: www.cyclecoach.com/.../is-the-map-ramp-test-a-valid-estimator-of-ftp

Thanks I'll try that workout when I get a chance. The ramp test I did on 12/24 was the FTP Guided Test built in to my watch. I just selected it from the Training menu on the Bike Indoor app.

The Garmin Connect Power Curve report shows that my 1 hour power record is only 221W for a long hill climb on an outdoor ride, so my actual FTP is probably just slightly higher than that. Something in the 230-235W range might be a good estimate. Although I think the trainer loses a few watts compared to using a pedal power meter on road rides.

"Although I think the trainer loses a few watts compared to using a pedal power meter on road rides." 
yeah, but only 1-2% in ideal situation. (depending on what kind of pedals do you have, dual or single one, and how accurate by default , used gear indoor, etc,) . 200w on Neo usually 202-203w on pedals/spider, the indoor and outdoor FTP is different a bit (or more), if somebody trains a lot indoor and has very good enviroment (large room space, fresh air, strong fans) then his indoor FTP is very close to his outdoor one (measuring with the same accurate powermeter), lets say 98-99% of it, if somebody is new on indoor trainer and has no good enviroment, it can be 90% or less, so a huge difference. unfortunately outdoor hard to find an 1h long steady climb (or flat without intersection or any problem), especially from sea level (Tenerife is a good place for it :)) . decrease your FTP in the watch at 230, and if You will have time, you will make the ramp test (honestly i always do this kind of ramp test indoor in the winter, 2 or 3 times maximum, at the beginning of the plan, and at the end of the plan and sometimes in the middle too)

Sure I'll try 230W FTP and see how it goes for a few days. I'm still fairly new to indoor trainers but I have a strong fan and good environment. Of course I prefer to ride outdoors whenever I can but it's been raining too much lately. I have Favero Assioma dual power meter pedals which are supposed to be pretty accurate. And I'm fortunate to live near a long, steady climb that covers 2139 ft (652 m) over 6.9 mi (11.1 km) so I can hit that as a repeatable benchmark whenever the weather is good.

Just noting the exact same issue still persists.  I have an FTP of 237W derived (.95) from a 20 minute sustained effort and today I was given 2x18 min at 265W - NOT POSSIBLE LOL.