Mirtazapine vs Amitriptyline? Weight gain and other side effects, what's your experiences?

Hi. English is not my first language, hope this still is ok to read! I need some help here!

I have isomnia due to antiepileptic drugs (I know it's odd, everyone else get tired from them but not me..). Tried different meds and combos, doses earlier in the day, and ofc I got perfect sleep routines.

But still severe insomnia.. lowering the ep drugs gives me sleep, but also brings back my focal seizures EVERY day, so not an option, the seizures are horrible and also they affect the brain real bad in the long run. Never had insomnia in my whole life before the ep meds (ep came just ~2 years ago, and I'm over 30).

I'm not depressed, no anxiety (actually really happy now as seizure free, it took a while adjusting to get the meds right).

So the insomnia is more like a chemical issue I guess...

Zopiclone works, but I don't want the tolerance problems so my plan is taking that just in periods as an extra sleep aid when I need to catch up.

I got prescribed alimemazine, it doesnt help me sleep, just making me groggy and still awake.

My other options right now are amitriptyline and mirtazapine. I'm freaking out about the risk for weight gain (former anorectic....) and also constipation, brain fog..

And another thing, if alimemazine did almost nothing, is it worth trying these antidepressants for sleep in my case? Or are they more like getting people to sleep because it makes their mood better? (I'm not having mood issues or anxiety as I said).

I'd love to hear your thoughts about this. And also experiences with these antidepressants for sleep.