“ESP jumping” is short term fix to a LONG term. It’s like trying use a band-aid to repair a bullet hole.
There are NO ESP’s that can guarantee “Inboxing”. As Service providers their only job is to DELIVER your emails messages. The same way that if running an ad on Facebook, they wont show your Ad to your most targeted users when your relevance score is low.
Email Inboxing is just the same.