I enter single line melodies into BIAB using a mouse even if BIAB will not be used further, because I also love the input method. Sometimes I use the keyboard, and for really difficult rhythm patterns, I'll use the BIAB qwerty keyboard entry on one pitch and adjust pitches later.

Couple of things to consider when using the keyboard as you describe:

You can quantize the melody after entering, which can adjust and 'tighten' the timings. Or, there is even the reverse, a function called humanize in some software including BIAB.

You can always slow down the tempo when entering, to increase the odds of playing it correctly. I do this. If you heard me try to play keyboard, you would know why.

Are you using a click track when recording in Sibelius?

BIAB 2025 Win Audiophile. Software: Studio One 7 Pro, Swam horns, Acoustica-7, Notion 6, Song Master Pro, Win 11 Home. Hardware: Intel i9, 32 Gb; Roland Integra-7, Presonus 192 & Faderport 8, Royer 121, Adam Sub8 & Neumann 120 monitors.