Learn Dutch: Is dit het einde van onze reis? - Is this the end of our journey?

You add the suffix -loos to the noun to indicate absence, making eindeloos (endless). Notice that it keeps the linking e. You could use it to describe a long trip: een eindeloze reis (an endless journey).

Why is it dit and not deze in this sentence?

When using this or that as a standalone pronoun with the verb zijn (to be) to introduce a subject, Dutch always uses the neutral forms dit or dat regardless of the noun's gender. Furthermore, einde is a het-word, so dit is the correct demonstrative pronoun to match it anyway.

I sometimes mix up ons and onze. Why is it onze reis here?

Reis is a de-word. The possessive pronoun for our takes the form onze before all de-words and plural nouns. You only use ons before singular het-words.