Honda Stepwagon RK5 2013 Sound Upgrade

Hi all I have just bought a Honda Stepwagon RK5 2013 and would like to improve the sound quality. The car came with an android 10.1" head unit with android auto and car play. I would like to replace the door speakers which are 6.5" and also power them with an amplifier. I would also like to add a small sub possibly one that can go under one of the front seats? How are the underseat subs are they worth it?

What would you recommend that is a good bang for buck decentish without breaking the bank. For the amp would it be best to get a 4 channel and 1 single channel for the sub or could I get a 5 channel for both? Should I also consider getting some tweeters that can go in the front where the dash meets the window?

I don't really know anything about car audio so any advice would be very much appreciated.

Thanks

Matt