Distance to our rented property for new construction on the plot

Nothing has changed about the size of our garden, only that our garden now ends at a house wall

That does concern the tenant very much. After all, he pays for the fully usable plot.

It can still be fully used, even if the tenant’s desire to enjoy the full use only with full sun exists. Although I question the orientation... it somehow seems to me to face SW towards the street. And if that is the case, then the rented garage is practically in the way itself. But whatever:
A fence and an empty field next door, practically a building gap, whether tenant or owner, shows even the blind that the “airy” days are numbered. Either I rent something like this and enjoy the numbered days or stay away from it and look for something else.
Personally, I would be more annoyed by the open gaps next to the house wall that take away the coziness of the garden. I would plant a hedge there and place the bench by the house wall. That gives warmth :)

looking for something else

For example with the garden by the railway line... neighbors won’t come that quickly... well... maybe a few ICEs...

Or by a river... with floods every few years...

There are worse things than a new neighbor ;)

I am glad to no longer be a landlord. There is too much potential trouble for the ROI. The landlord initially did nothing wrong and still has to deal with the tenant's quite understandable upset. Depending on how intense the emotions run, it is extremely annoying. For me, only AirBnB, furnished rentals, or ideally garages are options in renting.

A friendly architect once told us, "Renting is expropriation" - and unfortunately, he's not entirely wrong. But that's not what this is about.

I can definitely understand the frustration - having an open meadow as a neighbor was of course nicer. But as has already been mentioned several times here: it’s just the way it is that building gaps eventually get filled, which means less sun, neighbors who might look into the garden, etc. If the lease did not specify that the neighboring property was available for use, you can't do anything about the rent. Less sun due to neighboring construction is not a reason to reduce the rent.

As my predecessors have already mentioned several times: nicely design the wall, with the brickwork it looks quite nice. Plant a hedge for privacy and, if necessary, find an alternative place to sit on the property. If none of that makes you happy, then look for a new rental. You can take your time with that, you can still live there quite well. And if you come across a rental with a sunny garden in a secure, private location - then strike!