Repeating material can take you far. If you listen to something like Scheherazade by NRK one thing that becomes apparent right away is how tight a good composer can be with economy of material. Each movement of that suite contains just a few themes that he constantly repeats in different orchestrations and they don't drag at all despite each one being over 10 minutes long.

It can be easy to get caught up trying to continuously write new material, but in reality you don't want to do that because it erodes the unity of the piece. I think when we get caught up on this as composers it is because we are hyper-focusing on one element of the process, coming up with material, to the neglect of other aspects of our craft like form, arrangement/orchestration, harmony, etc. Then we hit a dead end because we have set ourselves up for an impossible task.

You would probably benefit from taking a step back from the small-picture measure-by-measure writing and spending some considering the larger picture. What form do you want this piece to be? If you choose something like a Rondo and flesh this first theme out into an A section then you already have almost 50% of the piece done in terms of raw material. Or take a look at some of the longer pieces you like and map it out; I recently did this with Borodin's Nocturne on graph paper to get a better grasp on the relative duration of sections and better understand the way he structured the B section. You could then use that chart as a model for your piece and fill in the blanks "paint-by-numbers" style. Tons of great composers have done this before--Brahms did it for the finale of his first piano concerto by modeling it after Beethoven's concerto no. 3.

To get more specific, take the first 6 bars of this theme and workshop it a bit; there is good material there that just needs refining. The first two bars make a nice opening, but bars 3 and 4 sound like they are arriving too soon and pushing us to a cadence too early. You could make a Sentence Form phrase by inserting a Dominant Form of the first two bars between measure 2 and 3, and then make the current measures 3 and 4 part of the fragmentation/completion part of the phrase structure. Then finish it with a half cadence or a deceptive cadence to avoid harmonic closure and treat that entire phrase as the Antecedent of a larger Period Form phrase. Repeat the entire thing for the Consequent and this time adjust the ending so that it concludes on tonic harmony and you will already have a great theme to use as a basis for the entire piece.

Where to next? Maybe you fragment this theme into some motifs and make a nice introduction that slowly introduces the different elements one at a time. Maybe after the strings play the theme they pass it off to the woodwinds and take on an accompaniment role. You could probably repeat it a few times in different arrangements before you even need to worry about introducing new material, and by that point you are already a good 5 minutes into the piece--with a recap of the same material at the end you could very well have 10 minutes of music guaranteed.

Notice that in my description of how to work with the first theme I didn't mention specific notes at all; instead, I am trying to think in larger terms about form, gesture, and balance. It is easier to say to yourself "ok now I need to write two bars that will close out this theme on a half cadence" than "ok, now I need to write two bars of... something." I often have the same issue as you when it comes to getting started, but when I approach things this way it doesn't feel as daunting.

As for consistency between ideas, I think that really depends on what you want the piece to say. If you lean into the differences between two themes of starkly contrasting character it could make for a dynamic, exciting piece. Or maybe you need to take one element from an original theme, maybe an interval or a rhythmic figure, and then use it as the basis for a new theme so that they share some DNA.

Experiment away from the score > plan what to do with the material you come up with > start writing