Pick a better tire, there are plenty of gravel tires that have either no real center tread or a continuous center line of tread between 700x35 and 40c. Tossed on your mtb they'll move at a decent clip on pavement and still see you fine on entry level mtb trails. I ride cross and gravel tires on plenty of mtb trails with no troubles, sometimes it requires going slower and sometimes it means I can go faster. An endurance or gravel bike are probably your best options if you want to go road. Touring bikes can be good but in my experience they tend towards slow and plodding while a gravel bike can have just as relaxed a position but feel a little more spritely. They're also not as typically available as gravel or endurance and often will come with bar end shifters which somehow still continue to survive even though an integrated brake/shift lever is better. All City, Poseidon, and a couple of other brands make decent, affordable gravel bikes that will do you well.