Any tips on how to insert the sponges with minimum hassle for you / discomfort for the girls?
Lube - empty some into a tub so that you can dip the applicator or hand into it. Slap a bit on the end of the loaded applicator and also slap a bit on the vulva entrance (means that the ewe doesn't get a cold shock when you touch the end of the applicator on her!). Insert slowly - if you twist the applicator from side to side slowly whilst you slide it in that can help. It gets easier with practise.
Have a bucket of hibiscrub solution so that you can rinse the applicator between sheep.
The applicators don't have a grip on them so are very slippy when you are covered in lube, I would recommend that you manufacture your own grip - I wrapped some string around mine near the top and over-wrapped in electrical tape, it made a huge difference.
Having someone to load the applicator for you is also helpful - you tend to have one dirty hand (which is holding the sheep) and one clean hand (for the applicator) - so a helper with two clean hands for loading is ideal. Or a helper to hold the sheep.
I wouldn't use the applicator on ewe lambs, a finger will do.
Note, you don't have to use the PMSG injection if you are doing it at this time of year (i.e. when they are already cycling naturally).
And defo keep your dogs away from the used sponges unless you want a sick dog and a huge vet bill!