Hi and welcome. The discs may play a part and i don't know what those discs do so to remove an uncertainty factor i would pick up some trusted discs. Disc vs hand and finger size can play a huge part especially with tall putters. Taller discs are prone to scraping the fingers with the bottom of the disc or sticking to the index finger. The smaller the hand and the shorter the fingers the worse tall putters are. And the better wider winged putters are for release cleanliness. There are pictures of different grips on the main page of this site and in tips section of www.innovadiscs.com
Throwing only putters will show the most mistakes brutally. What a putter won't highlight is throwing nose up which leads to high stalling shots with drivers. You might be surprised how small motions can ruin a shot and how many things need to be right simultaneously.
I disagree about shorter throwing distance than others not being so telling. It tells that your form lacks something and possibly your body control lacks something unless you are severely injured or atrophied in the muscles. I started out that way and 10 years and a plethora of learned tricks later i'm throwing over 400' at sea level which would translate to more at higher elevations. That means that there are so many things that can go wrong that only seeing a video would help to point out each area you need to improve at and how exactly you should change things. So can you ask the locals and can you post a video? Asking for advice can't hurt.
About good practice and playing discs: You shouldn't pick an overstable putter which would mask form errors and you shouldn't pick too tall ones for your hand size and not too understable ones that add their own twist on the flight on top of what you did. Ideally you want a disc that neither adds or substracts from your throw so that you can deduce what you did and what you need to watch out for on the next toss. Latitude 64 Pure could be a great medium in being grippable for all hand sizes and not being too over- or understable while driving. People have thrown putters 450' and most never toss anything that far. Your form begins to be pretty good when you get a calm weather flat land shot to above 250' and 300' is competent and 330' is great and 350' is open division upper half performance.
Ideally you don't want to spend a lot and want a disc that won't change too fast early from wear so that you can monitor your performance progress. And get to know the disc inside and out. Thus i would go with only an Opto plastic Pure at first and nothing else on top of your current discs. That does not mean that there aren't plenty of other better discs than the ones you have now i'm sure there are. At this point you wanna improve your form more. Then after that for nose up issue detection i'd get a fairway driver. Again straight not over- or understable.
Without a video i can tell that throwing up and down is so fundamentally wrong that it needs to be addressed immediately in training. That means you should throw stand stills only for so long in practice at an open field that you can consistently throw at proper unvaried height that does not dive down or lift up and stall. That means you need to copy an idea from pilots. Pre flight check lists. One thing you need to add and do for each drive and approach you'll ever make from now on is to do a mock up throw before the actual throw _and reach back at the same height from the ground as you have the disc rip out at_. If you manage to throw at a constant height and still get nose up throws you're gonna see the top of the disc not the bottom. That means that your wrist is up. If so you have two things to play with.
You can position the disc differently in the hand to get it more nose down. You can push the wrist down as hard as you can or earlier in the throw or both. The earlier you tense up the forearm muscles the slower the arm moves so it needs to happen fairly late. Don't be surprised if you need to push down with all your might to keep the wrist down. The anatomy of the bones in the wrist means that if the hand bends back from the acceleration coming back to straight the wrist raises. So you could try the hyper spin technique too. That means that you need to stiffen the wrist not to bend back (the position can feel like it's not bent but it can be only a high speed video can give the true answer as a European champion was surprised in a video shoot). The hand can snap forward 10 max 15 degrees and then you absolutely positively _must_ do a steely stop of the wrist or you can overextend the wrist and tear up tendons and whatnot in the wrist. So you must be fully warmed up and stretched and have at least half hours of throwing behind you before you can even start to do this at low power. Some lack the muscle power to stop the wrist fast enough to avoid overextension problems. Which can leave you injured for life. So you always must think if you even want to try this and that you're warmed up, limber enough and strong enough to pull it off and start slowly with few reps raising the intensity of the training very slowly.
Flat shots need running on the center line of the tee and planting each step on the center line. Anhyzer needs running from rear right to front left with the plant step hitting the ground to the left of the line you're running on. Hyzer is the mirror of that.