Hi Catherine,
It's been a fair number of years since I've ridden brevets (I lurk here despite not having ridden Rando events recently), but my prefered bike for Randonneuring and daily commuting would be what today is called the Endurance Road bicycle. Folks have successfully completed events on a wide variety of bikes, but IMHO there's a value to having a bike that translates our physical exertion into as much speed as possible (and thus distance covered) without beating us up (like a racing bike) or slowing us down (like a touring or gravel bike). On longer events especially, the toll of physical exertion accumulates, so riding an efficient bike makes a meaningful difference.
Though the industry name changes, this style of road bike...one that's optimized to be both efficient and comfortable...has been around for decades. Back in the 80s, the industry called them Sport-Touring bikes. There's been some form of this bike since (and before, for that matter...the design perists).
I've used a touring bike for commuting and distance events...a 1986 Miyata Six-Ten that, like your current bike, was made of a nice quality but heavy gauge steel. It was too stiff and noncompliant for Randonneuring, where we carry some stuff but aren't carrying really heavy loads. The Miyata rode like a truck. It didn't feel smooth until the loaded panniers were attached (so, many pounds of load). And since it was designed to handle a front load, too, the steering geometry was sluggish. I felt like I was fighting the bike.
My experience is that Gravel bikes suffer from a similar character. Since they're designed for off-pavement use (where the front wheel gets jolted by riding over imperfect surfaces), gravel bikes like touring bikes have shallower steering angles, more fork offset (higher trail), and thus higher wheel flop effects. In practice, this means it takes more effort to initiate steering, and once the wheel starts to turn, it wants to continue to turn (flop), and that requires more effort to stop the wheel turn. That's desirable when you have a lot of weight over the front wheel (which counters the effect) or are on a loose surface (where you don't want the front wheel to turn unless instructed), but it's undesirable on asphalt.
I rode almost all brevets using either a 1983 Trek 630 (frame...everything else had been modernized) and, later, a 2006 Trek Pilot (a first-generation Endurance Road bike...Trek's response to the Specialized Roubaix). Bike tech has advanced considerably since, but they are fundamentally similar to modern Endurance Road bikes...they have a mid-length wheelbase of around 1000 - 1010 mm for a 54-55cm/Med frame, higher stack, shorter reach for a more upright position, a *slightly-but-not-too-slack" steering axis angle that, combined with fork offset, creates about 57 to 62 mm trail for handling that's stable and predictable, but still responsive. They strike the middle ground between Touring and Gravel bikes with "slow to respond" steering, and racing bikes that are predisposed to turn when the rider gives an input (whether intended or not).
So, as types of bikes go, I recommend a relatively lightweight, not-too-stiff, efficient-yet-comfortable, stable handling Endurance Road bicycle.
Oh, and get good tires with a relatively low rolling resistance (e.g., Continental GP5000 or similar). It's hard to overstate the difference quality tires make when riding long distances. IMHO, tires are the best upgrade that can be made to any road bike, regardless of type.
Hope this helps,
Lin