Some internal parasites are becoming resistant to ivermectin, so that may or may not be effective, hard to say for sure. Safeguard is very effective for everything except tapeworm. Valbazen is also a very good wormer and the dose is a little smaller. I alternate with Safeguard and Valbazen. If you ever have an issue with tapeworm then a medication with praziquantel is best, you can get that in Equimax, or Zimectrin Gold horse wormers. Equimax gives more bang for the buck, but both will work.
It's good to have some basic wound care supplies, hibiclens, sterile saline, veterycin spray for flushing/cleaning, antibiotic ointment (plain neosporin, plain triple antibiotic ointment), vetwrap or cowrap for bandaging, gauze pads or telfa pads. I also keep calcium citrate +D (Citracal), a super B complex tablet or capsule, Vitamin E capsules and amoxicillin (you can get as Aqua Mox for fish without a prescription), Terramycin eye ointment (available without prescription), Corid or Toltrazuril or SMZ/TMP for coccidiosis, especially if you are raising chicks. Those are probably the most used items. Also a hydrocortisone cream, vaseline, and honey are useful if you have to deal with a prolapse. All manner of miscellaneous things can be helpful, it depends on how much you want to have. Oral syringes in several sizes are very helpful for flushing wounds and for giving medications (wormers) or fluid. Having supplies to tube feed or give fluids is helpful in those situations (
https://www.backyardchickens.com/threads/go-team-tube-feeding-updated-12-29-2019.805728/)
Scissors, bandage scissors, tweezers, q-tips, a digital scale to get weights on birds. I think as time goes by, you will learn what you use regularly and what is really helpful to have on hand in emergencies. Also a wire crate (or 2, or 3, or 4

) are useful for birds that need to be separated or protected, also good for using to break broodies. I've probably left out a lot.....
Here are some threads on building first aid kits, there are a lot of variations based on personal experience, so give them a look over, see what strikes you as useful to have.
https://www.backyardchickens.com/search/10891071/?q=Chicken+first+aid+kit&t=post&o=relevance