Unfortunately, it seems that it’s not easy to get modafinil any more unless you’re already on it. The other option is amantadine. It’s licensed for MS so you should be able to try it no problem.
There shouldn’t be a problem with you having a nap though - your employer is legally obliged to make “reasonable adjustments” for you to continue working. Having a nap in the afternoon so you can be more productive in the remaining hours seems very reasonable to me (and something that my employer let me do, many years ago when I worked full-time.) Definitely better to take an hour off to sleep and work at 100%(?) for 3/4 hours than work at 50%(?) the whole time.
Perhaps there other things that your employer could do to make your job less fatiguing too?