I already know what and how I want to make it, now all I want is use
standard parts as much as possible. With that I start drawing to see if
it fits and if my gears will be OK I have to gamble here because I don't
know how much work I can do -rowing.
Mind it is hands only but 2 way, push and pull.

Uh-oh!! This is going to ruin your rowing stroke. It not only has to be pull only, but the motion described by your hands has to be to pull straight towards you, lift your hands a few inches, move them straight away from you, then drop them a few inches. Over a table - under the table - over the table - under the table.

Otherwise next time you get in a boat, you'll suck, and have to re-train your hands.

...personally I go for something I can actually use on the
streets while still aiming to make it as efficient as possible within the
limits dictated by the practicality/usability.

Forget about pushing, then, and get a rest on the recovery instead.