Learn Dutch: Ik vermoed de reistijd is kort. - I suspect the travel time is short.

Normally, you would use the conjunction dat (that), which forces the verb to the end of the subclause: Ik vermoed dat de reistijd kort is. However, in informal or spoken Dutch, you can often drop dat. When you do, the second part of the sentence reverts to a normal main clause with V2 word order (Subject - Verb - Rest), giving us de reistijd is kort.

Is it acceptable to use this sentence structure in formal writing?

No, dropping dat and using the normal main clause word order is a feature of spoken Dutch and informal writing (like texting). In an essay or a formal email, you should stick to the standard form: Ik vermoed dat de reistijd kort is.

Does this trick of dropping dat work with any verb in the first clause?

No, it is generally restricted to verbs of thought and speech. Because vermoeden (to suspect) is a verb of thought, it works perfectly here. You will also hear it frequently with verbs like denken (to think), zeggen (to say), and vinden (to find/opine).

Could I flip the order and say De reistijd is kort, vermoed ik?

Yes, that is another very common way to express this in spoken Dutch. When you put the thought first, the tag vermoed ik undergoes inversion (Verb-Subject) because the entire preceding clause acts as the first grammatical element of the sentence.