Question New router or new AP

So, this is a follow-up to another thread i made around 2 days ago, but whatever was in there wasn't really important

TLDR; i currently have a router(ap combo) in the living room and 1 (pretty old and bad) ap in the kid's bedroom.
My apartment is not that big, but since the way its arranged, the master bedroom has 4 walls between it and the living room, and the 2 other bedrooms have 1 foot or more thick concrete walls, making the wifi slow in the first room, and non existant in the 2nd.
What i did was put an old ap i found in the attic in the 2nd bedroom, with the same SSiD, but i recently learned that roaming between the router and the AP is not something i can do.

This is not what i wanted and i decided that i need 1 good wireless point (I wouldn't get ubiquiti roaming gear, since its expensive, and my house is only 110 or so square meters (I think around 1050 square feet.) and i don't need such range)

Any router i get needs to be at the corner of the living room, since i have fiber to the home, and my isp would not move the fiber cable location.

So, to my question, for my use case, what would be best? a new ap, and put it in the middle of the house (right between the bedrooms, and with open view to the living room)
Or get a new router (I use the isp provided one, which is actually not that bad, but an aftermarket one would be better) and put it in the living room?
Even if you went pro-sumer gear like Ubiquiti there are no guarantees. Roaming is not a thing unless you also buy a controller, don't be fooled by marketing hype, your end device will just hang on to the AP it associated with unless it drops then finds a stronger signal. An AC Lite is really not that more expensive than a router, what is your current AP? You still may find that you are better off with two APs from the same vendor and turn off the wireless on the router. RF is a funny beast with so many factors, a site survey would really be the only way to give a definitive answer and even then is subject to change if your neighbors change their setup. Lighting, motors, kitchen white goods and your neighbors will all play a role. Sorry there is no magic solution on this one, I would suggest a quality AP in the middle of the house, as you said and see how you get on, this can always be relocated and added to if necessary.