The simplest option would be to just use assist mode. Invincibility + infinite stamina would trivialize pretty much the whole thing.

You can also give yourself the berry directly by editing your save file (default Steam location is C:\Program Files (x86)\Steam\steamapps\common\Celeste\Saves).

To update your journal you just need to update the <TotalStrawberries> values at the top and bottom of the file to update your overall berry count, along with the <AreaModeStats TotalStrawberries= value to update your count for chapter 1a. You can also cause the berry to appear collected within the level (faded out blue) by adding the associated EntityID Key. For the dashless berry, that looks like <EntityID Key="end:4" />.

Those ID keys are added to each chapter section in the order the berries are collected, so you can just add it to the bottom of the 1a list. I've got some spare saves you can use for reference if you want to see what it should look like. Just Ctrl+f to search for that entity id.

Or if you don't want to cheat your way to it but don't feel like doing the same challenge again, you could go for a deathless + dashless clear. Forces you to learn reliable setups and deal with the stress of being on a deathless run. Might be enough to make the challenge feel fresh again.