Both of these relatives are equally "complete"; the difference is that your first omits the relativizer—that or which.
A relativizer is a word which points backward towards a constituent of the main clause, the referent and forward toward a gap—a missing constituent—in the relative clause. It tells your hearer that the gap may be "filled in" with the referent, just like a fill-in-the-blank exercise or test.
Here are the unrelativized forms, with __ representing the gap:
- I think I should make a decision _ ...the gap represents an adverbial, something like "in this way"
- I think _ is right ... the gap represents a nominal, something like "this way"
Note that both are "incomplete" in exactly the same way: there's a missing piece. Except for that, they're both full clauses—both "complete".
We have a main clause "I make a decision in the way ..." where we want to add 1 or 2 to define that way. To link 1 or 2 we use a relativizer after the term to be defined—the referent—and . I'll use that for the relativizer, because you do in your #2; but which works equally well:
- I make a decision in the way that I think I should make a decision _.
- I make a decision in the way that I think _ is right.
Note that #2 here is exactly like your #2, while #1 has a that which is not present in your #1. However, the relativizer is only required if the missing piece—the gap—is the subject of the relative clause; it may be omitted in both of these sentences:
- I make a decision in the way I think I should make a decision _.
- I make a decision in the way I think _ is right.
Of course when you speak or write these there is no "_", no explicit gap, just an 'absence'. It is this absence that your hearer looks for to interpret what you have said.