I would recommend doing Ivermectin cattle pour on, 5mg per 250ml bottle.
https://www.amazon.com/dp/B00JAL3AAW/?tag=backy-20
Get 6 plastic ml syringes. Often pharmacies will give them away for free or very cheap. You only need about 5 to 6 drops per bird (20 drops is 1 ml, so compute math on syringe, whether 3 ml or 5ml, to figure 5 drops is so many bars...usually 1 bar in 5ml, 2 bars in 3ml) to make it easy).
Prefill the syringes. After dark, when they are on roost, apply syringe drops at back of neck or base of tail or even above vent (but not inside vent), whatever you can get to most easily. Since you've got a pre-filled syringe for each bird, you can move through pretty quickly with the least amount of flock disturbance. If possible use a head lamp flashlight (or hold one in your teeth if you can't find your headlamp) to give you both hands free. Simply place the tip of the syringe at the skin level and push plunger. There's no need to fuss, just touch the skin gently with the tip, push syringe, it will work its way down into the skin fully.
Ivermectin has shown effectiveness for mites and several common internal parasites, as long as resistance hasn't been incurred from over use. Since your hens have had no treatments, this would be a good all in one start.
Repeat in 10 days.
Otherwise, purchase Gordon's permethrin spray and spray after dark on roost all birds wetting feathers and vents. It's stinky stuff, so I don't prefer to use it. That will address lice and mites.
For internal worms, if you have the $$$$ Aquasol is FDA approved chicken wormer that is applied to the water (fenbendazole). You can also use goat wormer Safeguard, but it doesn't mix with water well. If you choose the much cheaper goat route, put in a bottom gravity flow water container where the birds drink from the bottom.
Most stuff on shelf is now not approved for chickens due to FDA rulings in 2018, so it makes it hard to find easy stuff to use.
I find the Ivermectin, which is non FDA approved, the easiest and broadest med for chickens parasites, both inside and outside. Be sure to pull any eggs for 7 days from family use (if there are drug sensitivities...Ivermectin is used in humans, and the amount would be minuscule). Pull from sharing or selling eggs for a full month after last dose to prevent egg residue possibility. (Studies show it peaks at day 7).
LofMc