The One+7 or Lg8 or Asus zen, Huawei p30 and of course Google pixel phones, these are flagship models that are or about the same as cost of a iphone or Samsung, get one of those. I don't know why, get a phone at the cost of the other companies, if you go cheaper you get cheaper, the phones above maybe don't have a pen, or fancy Ui 2.0 or services but your next shouldn't be apple. But to your point a non curve Galaxy note would be nice.
If you do want a phone without a curved screen, I suggest the Note5, even though it is a bit old. Or if you prefer something more recent, but not as premium or powerful, go for the LG Stylo 5. I don't know many premium phones with a stylus other than Note. That's why I always prefer Note. There have been rumors that a Note 10 Lite is in the making, best you can do is hope for a no-curve screen, which is likely. The cheapest S10: the S10e, has a flat screen, and it may make sense for Samsung to implement it on the rumored Note 10 Lite, but the Note 10 Lite may appear only in Europe. About the Bixby button: there isn't one on the Note10 lineup. There is only a side button, which can be programmed to activate Bixby or activate the power menu. I can understand why you don't like the curved screen, but I find myself loving it, even on my Note10+. It stands out, especially with the bezel-less display. If you really want a phone that is a do-it-all powerhouse with a flat screen, that is hard to come by. But if you want a glimmer of hope, the Note 10 Lite may offer some. Or a lot.