The Mac mini simply isn't a major priority for Apple. They've demonstrated they're not all that interested in tossing all their work and starting over repeatedly, so while I can believe the Mac mini will still look the same outwardly, an ARM Mac is probably going to be all-new inside and then stay as similar as possible from then on. They're not going to piecemeal it, they want something they can update and then forget for a year or two like they have with the current one save for spec bumps.
At this point, we're in uncharted territory with Apple making its own chips; we don't know how much tech is going to be shared between different platforms (are they only going to make 2–3 Mac chips, maybe just at difference performance thresholds based on power? Are some Macs just going to have iOS device guts? Are they actually going to have BTO processor upgrades or is it going to be like the 2018 Airs where you're just choosing RAM, storage, etc?) so it's impossible to say whether it "makes sense" for Apple to prioritize certain products switching to ARM sooner rather than later.
My gut feeling is that it'll be towards the end of the transition, honestly. The Mac mini is one of the cheapest products Apple makes, but they intentionally pivoted away from being the "budget" computer with the 2018 version, embracing its use as the "and everything else" Mac that's used as a server, secondary Mac, hobby machine, media digest, etc. That's a more professional niche like the Mac Pro, which I would likewise expect to be one of the last machines to switch (because pro users are going to be more gun-shy about new architectures, and because Mac's chips are untested in the real world with such massive performance requirements.)