I personally would add a power bank, and enough usb leads, if you take a suitable adaptor for abroad with several usb sockets on it then you and dad could share one, thus cutting down what needs taking. I find I often need a power bank to keep my phone going through the day. I use either a number or a fleece with a nice big pocket to hold it. I like a few wipes for travelling and days out too, I'd bring tail ends of packets, I don't use them much at home, I also try and take a pen or small pencil, you can always get a bit of scrap paper but sometimes you can't find a pen.
I'm not a frequent flyer but do travel on trains and coaches in the UK, so my needs are perhaps a bit different but I do need to keep weight and bulk down. I have dietary needs so will take some stuff for breakfasts that isn't easy to get in shops, and usually some.coffee bags. Obviously by the time I go home, I no longer have these. I like to have a bit of extra space for anything I buy while I go away so that sort of thing helps obviously not stuffing luggage too on my way. If where I'm staying has basic toiletries I'll use those, and just bring personal stuff I need. I would bring cream deodorant/antiperspirant decanted into a little plastic bag, also anything else I will only need a little bit of, mini toiletries, if I need to bring shampoo or shower gel I'd bring my mini solid bars on other occasions, all this depends on how long. I have saved almost empty toothpaste tubes, and used them all up or for just a short stay I have filled up the toothbrush protector head with toothpaste, I've even refused little plastic bags with a squeeze top, I save those as they have many uses. I bring teeny tiny little bottles of perfume, minimum make up, if any at all, again depends on what I'm doing, I'll be bringing some when I meet up with friends in a couple of weeks.
As others have said knickers are a key thing to take. I put aside any that need to go in the bin soon and maybe wear them travelling, and discard. I have invested in nice and comfy light quick drying pairs and hav recently discovered laundry leaves, I bring enough to do as much laundry as I will need, I'd wash out in the evening, you can tear bits off. If going for longer and I'll have access to a washer, I'll just do a wash half way through. I use a long tee shirt/a short light nightie. If I am at an air BnB or suchlike I'd bring a pair of little slipper socks, that after the air BnB that was no shoes in the house but had a very cold tiled floor
If it might rain I bring a poncho, which goes over my little rucksack, and I bring a light bag I can squeeze up for days out for bits of shopping etc. I also need sticks which collapsed and stuck out of the top of the wheelie suitcase.
I would dress up outfits if need be with different scarves and costume jewellery, both of which can be lightweight, and actually last time I needed to I popped the jewellery into my glasses case
Reading all these ideas, I might well invest in some packing cubes and see if they make the clothes packing easier. OP probably you won't like the ideas I use, which is no problem, but do consider the charger/power bank and maybe some of my ideas might inspire you to work out something that you could do to make things easier for you, and later on I'll be looking for packing cubes!