Both WhatsApp and Telegram let you mute notifications per chat and pin the ones that matter most, which is your first line of defense. The trick is being ruthless about it - pin maybe 2-3 chats per platform that are actually time-sensitive, and mute everything else. That way you're only getting alerts for stuff that genuinely needs your attention right now, and you can scroll through the quieter groups on your own schedule without feeling like you're ignoring people.
Beyond that, most people find it helps to batch-check conversations rather than constantly jumping between apps. Set specific times - like morning coffee, lunch, evening - when you actually sit down and catch up on all 8 groups at once. It feels counterintuitive but you'll actually miss less because you're reading full conversations instead of piecing things together across different check-in times. You could also ask the groups if they'd be cool with a quick summary message if something important happens while you're away, since honestly most casual group chat stuff is just banter anyway.
If a group is consistently time-critical and uses both platforms, that's a sign it might be worth asking if everyone would consolidate to one app. Sounds disruptive but it beats the constant context-switching. Otherwise, the muting and batching approach is what keeps most people sane with multiple groups.