I'll have to take your word for how Instagram has developed, since I have used Instagram for a total of about 15 minutes, and found it confusing and unpleasant. And it's definitely a good argument for expecting them to shift ground and go for what's left of Reddit, (maybe after they've mopped up what's left of Twitter, which might not take long, since Twitter is busily mopping itself up).
When I say "Instagram in a Twitter skin," I'm going off articles saying that Threads uses Instagram's algorithm, which seems a little less likely to change than the user interface/general style...? I can try to find my exact sources, if you like. It seems like Meta might have business justifications for adding a separate Reddit-replacement service, though there could be equally strong reasons to morph Threads for that purpose. I'm morbidly interested in seeing how it develops.