The Octatrack can be very useful, even if you use a DAW to sequence your gear. But a lot of the good stuff isn’t necessarily the more obvious stuff. Like being able to manipulat any input via the fader and the scene features. The fader can be used as macro to morph between parameters. From effect, over retrigger amount, to delay time just anything can be controlled with the fader, so it feels like some kind of instrument. I can morph a Bassdrum to the sound of a plucked string, when I use the comb filter. Of course those things can be done with a midi controller and DAW. Another advantage IMO is the ability to jam an mix patterns on the go, plus the fun and simplicity of hardware in general.
About the Virus. As „Buzz“ already stated, the Virus can be used without the dedicated plugin. The way to do this would be over MIDI, like you probably do it already with your OB-6.
There is one function that can only be used with the plugin. It’s the createion of custom patterns for the intern arpeggiator. Those arp patterns are cool because the can be used as manipulation source. Besides that, you can do everything on the hardware itself. I got the desktop version of the ti2, build quality is exceptional well done. I prefer the Virus sound more as sylenth, Massiv. Unfortunately you can’t edit the wavetables for the Virus, something that is very well done in Serum.
The DSI REV2 make sense, 16 voices of analog is something you can only emulate in DAW. (Diva plugin is pretty cool!). The REV2 has much more manipulation options and therefore probably a wider sound range as the OB-6. If I would buy a analog poly it would probably be the the Rev.2 Desktop 16 voice.
For FM sounds i would use FM-8 from Native Instruments. Not sure if the Digitone would add much to a DAW based setup, plus IMO there is still a few things that needs to be added per update. If you want a hardware synth for FM. The Montage serie is the synth flagship from Yamaha. Two engines, one sample based for natural realistic sound and a extremely powerful FM engine (8 operators, 128 voices, etc) Ther is even a version with less faders and buttons, cheaper converters, but much less the price of a Montag. But again everything could be done with plugins to.
If you not sure about semi/modular synthesizer, check out VCV rack. It’s a free open source based emulation of a Eurorack. Countless of free modules, even some 1:1 emulations of some high class Mutable Instruments modules or doepfer inspired emulations. Since the last few updates, it is possible control VCV parameters via your DAW.
If I were in your position i would try to find out what is missing most. Definitely try out gear in a store if possible.