... I'm realizing that the objects I meditate on, either the breath or an image, are extremely faint ...
You should move your mind to the object and then you should retain your mind on the object. As a beginner if you do not pratice the latter then the object will be less prominent. You should retain the object of meditation by continuously reviewing if your mind is with the object and regardless of whether it is or has gone to another object you should continuously redirect your mind to the object. After a while you will be able to retain the object and feel the object without issue. With regard to reviewing with the intention of retaining see p35, Knowing and Seeing 4th Ed by Ven. Pa-Auk Sayadaw
... it is hard for me to switch my attention to the sensations at the nostrils ...
For somebody starting out it does not matter if you feel any sensation or not as long as your attention is with the object. More you pratice the object becomes clearer and clearer.
This is like taining to be an athlete. You cannot be Olympic medalist the 1st day on training. You pick up endurance and stamina as you go along with your training. So is meditation.
1 )Should I continue with the breath,
... I'm truly focused upon it,
As long as your mind is directed at trying to feel the object, i.e., sensation round the mouth you are doing OK.
Once you start feeling the sensations prominently try looking closely at the object, i.e., sensations, until you can see arising and passing of sensations.
or should I switch to a visual imagined object?
This will not develop Vipassana hence it is best to stick to the above.
2)Should I keep meditating despite the faintness of these objects of focus
or is there a way to dispel this dullness outside of formal meditation?
Try to retain the object by continuously and persistently trying to retain the object and continuously review if the mind is with the object.
Are there any tips or techniques to do so?
If you are tense and not released then this becomes and issue. So relax your body and mind actively looking at tense or tight sensations that arise looking at arising and passing within them.
Though not related to this the content and references of my answer to this question has further information: How to balance an attitude of "effortless non-striving" with proper posture?