A light frame also carries no momentum, meaning it's less stable and more subject to minor changes from you or the road and wind.
They are meant for people who are pushing 90-100% all the time.

The plastic in the CFRP frame disintegrates over time, especially in the sun, whereas you can paint metal frames.

Aluminium frames are heavier but also very rigid. They are much stronger than carbon, so can take damage. Mostly used in mountain bikes.

Steel (chromoly) frames are great for long distance and comfort. They are tough, durable and the spring effect is good for soaking up imperfect roads like what we have here. It's also the strongest of the 3, so you'll find frames that are much thinner. They tend to be cheaper, but really good ones can cost the same as carbon or alu.

It's useful you read up about frames because this essentially is the bike. The rest of the components can be replaced.

A frame that fits your body is worth much more than anything else. Hence nobody can tell you what's best. If you can find a good bike maker, a custom frame is night and day different. Emphasis on good.

Buying used is a great way to get a good bike for less. Just make sure it's a good fit for you.

Don't get tempted by fancy components. In my case, I've got 21 speeds but only use the fastest one. Once you get strong enough, so will you. Disc brakes are great for feel and grip, but is an additional component. You'll only need it for carbon rims cuz the rims cost so much.

Sorry I can't talk about prices cuz I got my bike made a long time ago and haven't bought another since.