At what point does a road become too busy/fast to ride on safely?
Determining an acceptable risk level is really a subjective personal assessment, no one can tell you whether or not something is of an acceptable level of risk.  As such I will give you my judgement criteria for riding on roads shared with automobiles.


I avoid roads without a large shoulder or bike lane.  While many still advocate for taking a lane, the failure mode (e.g., a rear collision) presents too much of a risk in my opinion.
I avoid large speed differentials, I would qualify vehicles traveling over 50 kph as having a large speed differential.  This reduces reaction times as the closing velocity is quite high, which can lead to accidents.  I don't trust that most drivers are doing an effective job of scanning the road ahead of them.
I also try to avoid roads with much traffic, either by riding in off-times (e.g., 5 am) or sticking to largely unused roads. This further reduces the risk.
Finally, roads with good sight lines and acceptable closing speeds are preferred over roads with bad sight lines and higher closing speeds. (Thank you to @gerrit for the suggestion.)


I also ride away from traffic whenever I can so I tend to mix in a lot of dirt and asphalt.  For very quiet rural roads with low speed limits I am happy to forgo criteria (1) because there are fewer interactions with vehicles and you tend to stick out like a sore thumb in that scenario. But I also mitigate this further by only using these roads in good weather conditions (e.g., not at night in the rain).

In terms of line of clear sight I feel this only provides some safety as it still requires drivers to scan ahead and recognize objects.  Many do a poor job of this, or the search images they are looking for do not include cyclists so they can miss an utterly obvious rider at the side of the road.  This is why I put line of sight last.

From my criteria I don't think you need a critical assessment of your photo.  I would suggest looking at alternate routes (perhaps ones that are less direct) or look on sources such as Open Street Maps, as they have a good cycling path maps including off-road/dirt paths  that can be used to tie together routes so that busy and dangerous roads can be circumvented. 

At least where I live, this has been a good strategy.