In Bedrock, when you bonemeal a single-high flower, it will cause more flowers of that species to grow around them. There are very few farms using that mechanic to farm flowers of one type as needed yet, so I came up with my own.

I plan to put this in a tower with buttons on the first floor to activate the compartment I need right then, which is reflected in some of the design, especially the hoppers.

The video is in German, so here's an explanation of how it works in English. Note that the video has an error at 9:00 when I realize I confused front and back and made a cut to repair the farm (I explain this, but in German). Other than that,I think it's fairly obvious what I'm doing from the pictures.

The farm has one flower of choice on the base floor. It is surrounded by dirt and, on one side,a light-giving block (glowstone in my case). This is at the center of a walled-in 7x7 area of grass. On top of the flower is a dispenser with bonemeal facing down. A line of hoppers points into the dispenser to deliver more bonemeal as needed, touching the ground above the lighting block (to block as few blocks for flowers to grow on as possible).
A line of hoppers along the front of the farm and two hoppers in the far corners collect the flowers produced.

Two blocks above that dispenser is a second dispenser, this one facing the far side of the farm with a bucket of water.

The redstone splits in three to do these things in order: Bonemeal the flower, acticate the water, deactivate the water.
Four repeaters on the last line so the water is dispensed long enough to carry all flowers to the hoppers.

Rates vary between 8 and 11 per activation, sometimes up to 14.
Dandelion and poppy will not be pure, as they have a chance of spawning each other. Does not work with wither roses or two-high flowers (which only give one flower per bonemeal, anyway, so why would you ever farm them?)
Bedrock 1.14+, won't work in Java or older Bedrock.