If the aircraft are 5000 feet AGL or higher, the line of sight should be fine to receive them 100 miles away with the antenna as-is on you roof. Going from 25' to 30' AGL is about one mile further distance.
I use an AT-197/GR mil air discone that is one foot above my roofline and receiving 100 miles away is not a problem. I RX much further than that actually. I use LMR-400 for the 75' run and a PAR mil-air filter for 225-400 (I have noise here). I use a preamp, but if I could get my hands on an Angle Linear LNA for 225-400 MHz I would be set. While your terrain and mine are not the same, I'm figuring that log periodic really helps, but I wonder how much it hurts due to the directivity with that many elements. It's like trying to find a plane in the sky using a telescope when one could simply look up and see it. It does get a broader view further away, but sounds like an A/B comparison project is needed with a discone.
Do you have a structure or terrain nearby that is in the way/higher than your antenna and where you want to RX?
Do you have a discone on the roof you can switch back and forth to check the RX?
Have you swept the LP from 225-400 MHz? I'm figuring it is fine because it is a LP, but best to check.