I've been watching lots of youtube videos, but they seem to mostly be clear sightlines, either FPV but not going far at all, or a few that are not FPV and go far away, but still with a clear sightline.
What should I be considering, if I'm looking for a drone to fly in a hilly, wooded area, up to 1500' away? Not necessarily a specific brand recommendation, but general things that will tell me if a drone would have the signal strength for what I need?
Purpose: check fencelines for fallen trees, and sometimes check for trespassers or other issues. I was considering FPV in case it is easier to navigate through the woods and also see the fenceline, but that might not be necessary.
Yah there are no guarantees when it comes to signal strength because the answer is it depends largely on the environment which itself can change over the seasons. Basically if you have a directional antenna it will help to focus where it is picking up the signal from and can get quite a bit more distance but is a relatively narrow "beam".
I think the suggestion for DJI digital is probably a good one but you might consider putting a gps on the quad and learning to use iNav if you want it to just fly some pre-planned route and not necessarily worry about if the fpv link drops for some time (iNav isn't trivial to setup but lots of people use it so isn't insurmountable either). There are also proprietary solutions that could do the route navigation (I would presume skydio or dji quads can be flown this way) but personally like to stick with open source things as much as possible to keep it easier to fix or swap components without running into walls.
In any case vegetation eats the signal pretty quick so staying high in the air and keeping as much of a clear line of sight as possible is important to keeping both video signal and control link. Having a hill between you and your quad is a surefire way to lose both links . Also you'll want to stay clear of any tree lines especially when starting off learning if flying the quad manually. Also if you do decide to learn to fly "manually" it can be a lot of fun but takes quite an investment in time and effort to get good enough to fly confidently. Using a simulator like velocidrone or liftoff (both have free demos) is a good way to build up skills without breaking the bank (along with watching tutorials). If you have a few days of sim time it's worth months of crashing and rebuilding
If flying via gps control and waypoint navigation can skip on learning to fly but for me it's my favorite hobby so wouldn't suggest skipping on it.