You can easily get a 65' basic LED from any manufacturer for $600. Costco has a nice LG for that price.

Consider brand. Samsung and LG are top. Sony is mid. Hisense and TCL are budget brands. With these brands you will get more features for less, but a TV with comparable features might not be as good.

Besides brand there are two basic considerations.
Display type.
LED, or LCD is the most basic.
QLED (quantum dot) has brighter and better colors. Different brands call you his by different names. LG calls it QNED. But it all means the same thing. Better color.

Within these two types of displays you can get more contrast with different types of backlight. Full array means the backlight is always on so your blacks are more gray. Micro-LED has hundreds of back lights so you can get better contrast.

OLED (Organic LED) is considered the best display because it has the best contrast. Each pixel has its own backlight. However peak brightness for OLED is not as bright as QLED, so if you are in a room with large windows facing the TV you may not want OLED.

For your price range you can probably get one of the following.
(1) a budget brand QLED, or micro-LED TV,
(2) or a premium brand basic LED.
(3) Or a giant 75" or bigger LED budget brand TV.

As others have said OLED is going to be out of your price range.