The platform you're actually looking for is probably Quizbowl.org or the NAQT (National Association for Quiz Bowl) online events - they run casual tournaments throughout the year specifically for teams that want to compete without the rigid structure of a full season commitment. You can also check out local quiz bowl forums and Discord communities where people organize pickup matches. The nice thing about these is they use actual quiz bowl questions so the difficulty and format stay consistent with what you're used to in competition.
If you want something more flexible and less structured, Sporcle works fine for solo practice, but here's what most people don't mention: it doesn't really replicate the team dynamic or the speed-based pressure of actual quiz bowl. You're essentially doing individual trivia quizzes rather than training for the format you compete in. It's fine for breadth knowledge but won't help much with buzzer timing or team coordination.
For the best bang for your practice time, try hosting or joining informal team-based competitions through platforms like Zoom with question sets you can find online - there are plenty of archived quiz bowl questions available. Some people have had good luck just reaching out to other schools in your region and setting up casual scrimmages, which lets you practice against real competition without the league overhead.