A dipole antenna is a fine antenna for what it is (a unity gain antenna in free space) and poor is a relative term. The important thing is how much signal is seen by the antenna for the station(s) of interest. A dipole may meet all the needs of a person or it may not and a directional antenna with gain may be needed. The amount of gain and HAAT needed will depend on the amount of signal available to the antenna.
Not everyone, especially when HOAs are involved, will have a choice to use an outdoor antenna, let alone a high profile directional antenna with a rotator.
You mentioned DX in your post. This can mean different things to different people. Extreme DXers go for the most signal possible, paying attention to each fraction of a dB. This usually means the use of purpose built FM broadcast band directional antenna and a rotator.
The j-pole antenna is an end fed, half wave, unity gain antenna in free space.
They can be a fun construction project, but they do not violate any laws of physics in terms of performance. There is nothing magic about their performance. A lot of people have built this antenna and it seems to work okay for the most part.
There is a some variation in the accuracy of the construction plans found on the internet.
The use of a correct common mode RF choke at the feed point can improve the performance of this antenna.
The pick up pattern of this antenna is not completely omni-directional and without the use of a common mode RF choke it is even less so. The outside of the shield of the coaxial cable becomes part of the antenna and the position of the coaxial cable can change the reception pattern. Local electrical noise can be picked up by the outside of the shield of the coaxial cable.
In general TV antennas do not work as well as purpose built FM broadcast band antennas, but might just work okay depending on the available RF signal strength.
TV antennas such as those in the link above do not necessarily provide good FM broadcast band performance. They usually have only one element for the VHF frequencies (not tuned to the FM broadcast band) and achieve their claims of gain by the use of a broadband RF amplifier.
These amplifiers boost a very wide range of frequencies other than the FM frequencies and this may cause unwanted issues and may add unwanted noise to the signal.
FM Fool gives the expected RF signal strength in numbers and as mentioned you can compare the numbers to the quality of reception at your location.
"B) listen to a station ~40miles away" Look at the signal strength available using FM Fool this will help you decide how much antenna you may need. Remember this is only an estimate.
"A) increase the FM signal" and "E) Get the best range out of a passive antenna" Directional antennas in general give the most gain (range) but may require the use of a rotator.
What works well for one person may or may not work the same for another person in a different location (even just down the block) Without an RF sight survey and actually measuring the RF field strength and direction of the station(s) of interest, or experience in that location, it is all arm chair conjecture. What works well for one person may or may not work the same for another person in a different location.
What are you using for an antenna at this time and how well are you able to receive the station of interest? How well can you receive this station on your car radio.
