I often like to use stock plugins for OSC.
There are countless reasons for this, including that I share my project files.
Unfortunately, Reaper's stock reverbs are harsh and unnatural in my comparisons to many other reverbs.
The simplest way to make Reaper's reverb sound good is to load a free reverb impulse response, of which there are many online. This is thanks to a fairly recent Reaper update.
I suggest using a simple reverb impulse response.
There are countless reverb impulses which would be completely unacceptable for the OSC because they are not a simple reverb.
There's a conspiracy theory that many reverbs are actually impulse response reverbs masquerading as something simpler, because most people aren't going to delve into this subject.
The spirit of the One Synth Challenge
was "friendly" musical composition using a single synth for all sounds, in real time, while not using additional modulating plugins.
Disqualification was extremely rare, with several winning entries using gratuitous EQ filter automation, and amp sims. The former would be absurd to limit, and the latter seeming to only make a significant difference if they use heavy distortion.
Things have clearly changed.
Cabinet simulation includes the frequency curve of the microphone, the relative location of the microphone, the speaker, the cabinet resonance, and the room, which can all be sculpted and replicated with a single precise equalizer. There are subtle nuances that occur depending upon when the frequencies interact with several forms of clipping and distortion, most of which are negligible, as long as there are multiple stages of saturation so that it's not perceived harshly, and that the mids are emphasized prior to significant clipping, so that the lower frequencies don't eclipse the frequency spectrum with excessive harmonic content.
Enjoy some of what Jim Lill brilliantly has to share:
https://youtu.be/wcBEOcPtlYk
https://youtu.be/y8GiF-GVLgg
Apologies, I don't have the luxury of responding to any further dissent from you regarding these matters.