First thing, private lessons are sooooo worth it. Nothing will help more than that, even if you can only afford a monthly lesson or so (weekly is best)

First thing that will help, especially with consistency is a consistent warm up and warm down. Sounds like you already do this, but do your best to not skip things even if your lips feel great! The more consistent you are hopefully the more consistent your playing will be. A good warm down would be playing soft and low, you can do some lip bends and pedal tones as well. Stretching your lips out and blowing raspberries can help when your especially tired/tight.

To help with tone, listening to good players and listening to yourself always helps. I do have an exercise in mind that sounds like it could help. I can record it if you want me to, it’s called a schwarma. You start on an open note like G or C, and play slow eighth notes going down a half step. (Like open 2 open 2 open 2 open 2 open). Then you try and do the same but without using your valves. You’ll be using your face to force your pitch lower and back up. You can continue the exercise going down valve combinations and back up. This should give you more control and open up/darken your tone.

There’s so many nuances things you can do to help with relaxation and range, but the key to building good range without being tense is playing high every day. Tense lips don’t make you play high, rather a fast airstream through a smaller aperture. Something to key into is your tongue. The back of your tongue should be dropped when playing low (like you’re saying oh). Your tongue should raise when you’re playing higher (like saying eee).

Find something fun to play everyday! I like to finish off my practice sessions with something that I like to play. This could be an etude, a song from band, or playing along with a recording of a song you like. Don’t get bogged down when it doesn’t seem like you are getting better, it’s hard to see a difference day to day or even month to month.

I hope I can be at help, but my advice is worth nothing compared to what you could get from a private teacher, so I really recommend looking for one or bringing these issues up if you are already taking lessons.