A simple way to allow players to play a single character is to abstract the seasonal improvement system. Instead of requiring uninterrupted lab time, just allow characters to perform laboratory or other seasonal activities without any penalty from adventuring or otherwise performing other actions during those seasons.
If you want a more immediate, adventuring-focused game then simply remove all sources of xp other than Adventure xp, and vastly shorten how long it takes to perform activities such as transferring vis or designing a new spell – have it take a week, say, rather than a season (and something that would take multiple seasons instead takes multiple weeks, etc).
Keep in mind though that a lot of the charm of Ars Magica comes from the idea that wizards are cranky tower-dwelling misanthropes who largely want long uninterrupted periods to tinker with things in their laboratories. The mechanics naturally leading to wizards who either have some specific focus that drags them out of the lab, or who only leave the lab reluctantly in response to disaster or need for further materials, and thus model mythological/fantasy wizards, is a big part of the draw.
Typically the storyteller will make use of timeskips to create scenarios where magi have time to perform their various lab activities but then are present for the next ‘adventure’. There’s nothing to say that you could not use that mechanism to avoid companions or grogs being PCs entirely, yet still have the covenant/timeline progress. I’ve played in games where no-one really used companions – although, the magi being played were of different ages and levels of power (and sometimes apprentices) so there was still a bit of a ‘these are the magi, and this is Tim, the 22 year old apprentice mage who knows one spell’ vibe going on.