If you have "scratches" on your back and Ivermectin causes a tickling sensation, I would consider that a reaction to it by whatever is there in your skin.
A colleague who got scabies in Bali from the bed, said her back broke out in ooze and inflammation after taking Ivermectin.
Sadly, she stopped taking it regualarly and she still has them hatching a bit, but very much reduced.
If this is a recent occurrence I would encourage you to choose a regime after reading what other Iver users have done and continue it for at least three months. Possibly tapering off for a further three months. At least.
I dont think any of realise the LENGTH of time we need to treat for rather than the intensity of treatment.
In your position with your experiences so far, I would go with Ivermectin.