The best answer - if at all possible - is to add 'some structured cabling' - aka CAT6/UTP, so that where extra APs are still required, they can be optimally located. Obviously, there's expense and upheaval involved - how long are you going to live there?
It's very hard to know, exactly what kit would be best for your particular house. You've not said what the requirements are wrt performance (as in, you can be connected, but with lack-lustre throughput). [My lower limit is 25Mbps, for 4K Netflix - because I don't do any large transfers between wireless nodes].
You probably need to some sort of survey of the signal levels your current kit is achieving in different locations. There are limits on transmit powers, so I'm guessing there's not much difference between manufacturers in this respect. (The connected equipment (i.e. your phone etc) is limited and fixed anyway ... and the conversation needs to flow both ways).
There are many apps you can use to do your survey - but the best device to use is an old Android phone/tablet, running Android 6. (Later versions limit the no. of times an App can query the Wifi - so they don't update properly, unless you manually kill and restart them!)
Try this site:[url] https://www.metageek.com/training/resources/wifi-signal-strength-basics/[/url] for inspiration.
Just one thought about your existing extender - have you sited it at the mid-point, between the Router and the clients? Some people (my sister!) have been know to place it close to the problem Wifi client and find that doesn't help at all!
