No suggestions but I wouldn't think a hollow core door would be acceptable as an exterior door.

These doors usually have wood bracing in certain areas before they clad it but it is only 1 1/2-2" thick along all edges and around the window. If you have holes already since a spring closure is best I would use some Bondo or similar type very hard product to fill the holes then insert the bracket when the Bondo is wet and let dry with the screws imbedded in it to have it hold tight.

No window boxes and planters on a drip irrigation system:

No window boxes, and IN THE GROUND soft landscaping below.

Or ? Show how you plan to use the interior as guest space, on graph paper, all dimensions, and a reason the door can not swing to the inside.

If you double or triple the width of the stoop there will be room to the right to put a door stop of some kind.
Try a large sturdy planter or affix a railing on that side.
No window boxes as Jan says...they are an ongoing headache to maintain.
Do show us your final solution!

This what is often used to restrict the opening of doors on outside accessory buildings.

add a proper landing level with the door then create the step to the landing. make the landing big enough so you dont have to step off it to open tye door. that landing will be close enough to add a stop on it and it will create a much more graceful entry.

BeverlyFLADeziner is on it above.

If you use the chain and spring shown above, I'd add a plate to the outside of the door and bolt through it and the door to attach the inside door bracket. Go as high as possible on the door as that's where the most structure is.

These doors are foam (styrofoam) filled with wood around the parameter. Screws into the thin steel, and even screws through the steel and into wood will tend to pull out.

A hollow core door is not an exterior door so start there I agree a much wider stoop is a must and then you could use this type of doorstop

Agree the stoop should be wider - I’d go old school and plop down a vintage iron door stop on the right side of the stoop (the wider area would keep it out of the way) there are a lot of charming ones on Etsy and local antique shops

Get corner rubber bumpers for the window box.

This is something different, and adds to the country shed theme.

EXAMPLE: https://briansfurniture.com/product/horse-hitching-post/

It could be placed close enough to the shed and the base modified to provide the door stop.