My suggestion to this: as somebody who normally plays tablet+kb and wanted to learn mouse-only in the last two weeks, I think what you should do is, play a lot of maps that were the hardest maps you could do with your usual playstyle (mouse-only) with the new style you want to learn (mouse+kb) with NoFail activated (add SpunOut if you don't want to hurt your stats) and just aim to have maximum accuracy on your runs. Even if it feels impossible, try to do your best for every single note. NoFail is key here. Just play a map fully with the new style, then move on to another map, don't retry the last one, and after you've been over like 30-50 maps start re-playing ones you played earlier and see your improvement in accuracy rating over previous runs. Ignore max combo completely, only pay attention to accuracy.
In 5-6 hours you should start feeling comfortable with the new style, and in just over a week you'll be equally good with it to your old style. At least this worked for me. Good luck!
On a side note, you don't need a keyboard to get better. You can stream perfectly well with mouse-only style, you just need a good mouse. I recently bought an A4Tech mouse to replace my logitech one and just doing this increased my maximum mouse-only streaming speed by like 50% (tested it). If you ARE going to play with keyboard, having a mechanical one can help, but it probably won't make any real difference up to 220-240~bpm maps.