If I suspend the plan on purpose (by removing the autopay) for a longer time and make a payment before the 90 days are up to re-activate my plan, will all recent add ons like bonus data and talk still be available?
Add-ons will still be there if you activate your service in time before account closure. Some bonuses might have conditions that stated that they will be lost if the account goes into suspension.
Add-ons that you purchased will be there
Most of gifts will be there too, but those with eligibility if being active won't, I've seen it disappear once before. If you were suspended through no fault of your own (stolen phone or renewal glitch) you can ask mods to do something about it though.
Those that you earned will also be there, you just won't collect them while suspended.
And some people thought they can use add-ons wile suspended to save some money and use it up, but that's not possible as you need account on working order to use services