(It seems pretty weird to me, but) I'm willing to accept for the sake of the argument that you might be able to get away with putting a parenthetical preamble at the beginning of a sentence if the sentence proper began with a proper noun or with the pronoun I, since in that case the sentence would retain the appearance of completeness if the parenthetical were cast aside.
But fundamentally the notion of leading with parentheses strikes me as sending a confusingly mixed signal to readers. What is the point of putting "in my opinion, at least" parenthetically, anyway? I imagine that it serves to acknowledge that the assertion made elsewhere in the sentence is merely the author's opinion—something that readers may well have figured out on their own—but to do it in a way that subordinates the enclosed acknowledgment to the open opinion. Then why would you put the segregated acknowledgment on klieglight display at the start of the sentence?
If you think that acknowledging the opinion-based nature of the assertion that constitutes the primary content of a sentence is front-of-the-sentence material, you should be willing to present that acknowledgment without parenthetical hedging. Conversely, if you feel strongly that the acknowledgment needs to be subordinated (within parentheses) to the assertion, the acknowledgment has no business occupying the front of the sentence and stealing the thunder from the assertion that follows it.
This at any rate is how I would argue the case against opening a sentence with a parenthetical to an author who thought that doing it would be a neat idea. I wouldn't get to the question of whether the first word after the opening parenthetical phrase should be capitalized because I would deny the logical validity of leading off a sentence with a parenthetical phrase in the first place. And of course, when you're deciding how to handle a logically invalid construction, you can proceed however you want, without fear of invalidating it thereby.