I wouldn’t have noticed the different windows if you hadn’t pointed them out. Especially since the sizes and shapes of the windows are very nearly the same.

Are you interested in updating the landscape? Or in enlarging the door stoop? (Since you say you own this “flat” and someone else owns the upper “flat,” I’m unsure about ownership rights of the yard and entry door. “Flat” isn’t a term I’m familiar with.

A flat is an apartment in this case like a duplex but up/down. As for making the building look better I am not sure of the arrangement in this case. I would concentrate on decorating the front entry with maybe a nice sized black planters at the bottom of the steps filled with seasonal plants , Maybe a pretty wreath on the front door and larger house numbers in black.

You could paint the area around the front door, the top and either around the bay window or the top window in an accent color.

My last home before and after. Painted trim and added accent trim to garage.

Not a pro. Can you call the company that made the upstairs windows and see if they sell the snap-in lite dividers?

ETA: Oops! I see that you only own the bottom half. I wouldn’t worry about the windows if I were you.

Since there isn't much you can do to the upper level of the building I would concentrate on more cosmetic additions and landscaping. Is landscaping something you can change?

Yes I can make changes to the landscaping.

Around my way, there are plenty of old Victorians that had all the trim removed and vinyl siding put on. They look a lot like your house because the siding tends to be put on top of older siding, making the windows and window trim less prominent. I work in a building built in the 1700s. It's last incarnation was a sorority and it's current is an office building. You'd never guess it was truly old. You might think 1950s, because the interior is worn and not up-to-date, but you'd be off by 200 years. If they redid the interior, you'd never know.