I’d hate to be accused of balking, but I must balk just a little (all in a spirit of camaraderie and motivated by our shared desire to produce excellent notated music, of course ).
The fact of the matter is that, regardless of what an individual composer in the 18th century may have themselves privately conceptualized, or even if they all thought the same thing, today the correct notation is a tie, not a slur. The fact that a slur “makes sense” to modern composers/engravers who happens to lack the ability to input the correct notation strikes me as perhaps a little bit conveniently ad hoc. (And it’s worth pointing out that if we’re able to think of, e.g, four 16ths that are tied together as actually being a quarter note, the way Dorico smartly and arguably rightly treats such things, and adding a tremolo to a note allows us to do something of a converse operation, that is, if we want four 16ths in a roll, we can “tie them together” into a quarter note and put a two stroke tremolo on it/them, I don’t see why adding a fifth stroke should be treated different, i.e. slurred instead of tied.) It’s also worth pointing out that they’re literally called “tied rolls” and “untied rolls”: Concert Snare 10: Tied and Untied Rolls / Vic Firth Percussion 101 - YouTube
In any case, while we wait for a proper solution to the issue, if one is to come, I wouldn’t recommend using a slur (unless you need playback to be accurate, in which case, slurring can be one way to go, but with the necessary manual visual adjustments, this can be somewhat time consuming depending on how many you have; I have definitely gone this route though). Without adjustment, slurs strike the eye, at least to me, as not much different from novice Finale scores I’d see sometimes where the composer didn’t know about the slur tool and so they’d just use only ties instead (sounds preposterous, but if you only ever need a slur to connect a note to its neighbor occasionally, as in some choral music, it totally makes sense).
If you don’t need playback to be accurate, the best looking route, I think, is to tie, set “Single stem tremolo” in the properties panel in Engrave mode to “None” (make sure “Set local properties” is set to “Globally”), and then create a custom playing technique that looks like an accent to use as your second accent. I think you’ll need to manually adjust each one (just the accents, that is), but that’s usually not too tedious. If you need both proper engraving and proper playback and don’t want to go the slur route, I’ve also gone with duplicating the instrument in question, copy-pasting the part, removing all the ties in the duplicated one, and then simply not including this “playback only” instrument in the full score. Also, if you’re doing pitched percussion, I think you can put the finishing stroke in a separate voice, bypassing the need to switch to Engrave mode and allowing for additional articulations in the tie chain, which is nice, and I think it even might play back correctly.
I will say one last thing, which is that I think the request to be able to draw a slur that looks like a tie for this situation came from me (though probably seconded by @Robby_Poole and/or a few others), but I don’t currently hold that to actually be a great idea or solution. The Dorico team is generally resistant to implementing stopgaps/workarounds, which I think is totally legitimate. As it currently stands, there are ways (if perhaps slightly more time consuming than they could or should be) to either get proper engraving or proper playback or both from this situation. And ultimately, I don’t actually want a stopgap (which very likely could create problems of its own); I would rather that Dorico simply understood percussion notation, a desire I imagine is shared by the team. As such, I think it’s probably best to just wait for them to make whatever adjustments need to be made to the program for Dorico to be able to properly handle this type of notation (and hopefully make entering it a bit smoother; having to switch to Engrave mode in a lengthy orchestral score can be… slow going…). Obviously, it would be nice if that’s sooner rather than later, but I think it’s fair to say that they’re at least aware of the want/need from the community and when a fix does come, it will likely be everything we need it to be.