Hi, Sark. Welcome to the Guild! Can your trepidation be fear of messing up - of irretrievably getting the setting sideways for what is obviously a project important to you? Rest easy - a fantasy world is forgiving, and readers *want* to believe; the majority won't be nitpicking.
I tend to do worldbuilding from a little random this or that, then let things fall where they may. But the particulars you list could plausibly be part of any number of worlds. How about taking each of your desires and listing a few preconditions or parallel features. Say, for karst you need limestone - perhaps a former seabed ages since uplifted and eroded. For extensive rivers you want large landmasses, with significant parts in the tropic or temperate bands. Your concern about tree species - just parallel some earthly continent and say that coastal rainforest has sequoias, firs, and redwoods.... or nonspecific big conifers. The alpine slopes may have aspens, runty windswept pines, and in valleys, tall straight pines suitable for ... building, bridges, masts - pick a specialty.
If you already have a need story-wise for certain biomes to be near one another, look for an arrangement of mountains, plains, and latitude that makes such a setup, and throw some together to get at least one plausible arrangement. If you have extensive story background built up though, your situation may be beyond my serendipity-driven worldbuilding flavor.
Maybe you need to adopt a page from software engineering - the principle of trashing version zero. The first setup you generate might be most profitable as practice. In any case again - it's a forgiving practice, this devising of worlds. If you were mapping reality, a bad wiggle sends a border where it really isn't. In fantasy mapping an unexpected wiggle is just ... different. Not wrong or bad. You'll do okay!