Reddit 1-3 PM UK time works best for me for English language subs with US heavy audiences. That's 8-10 AM Eastern, which catches US workers checking Reddit at start of day. Tuesday through Thursday only. Monday morning is noisy, Friday people check out early, weekends are dead for professional subs.
Twitter inconclusive. I've posted the same content at 8 AM, noon, and 8 PM UK and gotten wildly different reach for reasons I can't attribute to timing specifically algorithm variance dominates.
LinkedIn 7-9 AM local time to the audience, Tuesday and Wednesday best for B2B. Evenings and weekends are reliably dead on LinkedIn because it's a working hours platform.
The thing I've stopped doing is chasing optimal times on Instagram. Reels get distributed when the algorithm decides they should, and it's largely disconnected from posting time in my experience.