HandBrake

Constant vs. Variable Frame Rate for streaming devices
I am currently progressing through ripping/encoding all my DVD's and Blu-rays that I don't have digital copies of, to stream from a Windows PC to various devices (tablets and Firesticks w/ VLC, Plex, and whatever Roku has). So far for the DVD conversions, I am using Same as source framerate with Variable Framerate selected. Am I playing with fire with this setting for framerate or is this a fairly "safe" setting, generally compatible across most "stuff"? I don't want to get too far into my collection doing the wrong thing and wanted to get some expert advice and experiences of others. Thank you in advance.

Constant versus variable probably won't affect compatibility with your devices. Most of HandBrake's official presets use peak frame rate, which is variable frame rate that is limited to a certain maximum. So, peak frame rate 30 fps won't change any video at or below 30 fps, but would reduce higher frame rates, like 60 fps, to 30 fps. It's a good idea to use this and set the peak frame rate to the maximum your devices handle, often 30 but sometimes 60 fps depending on the device.

It's fine unless you have some real crapgadgets, but they may have more limitations than variable framerate.

The age of your devices is the determining factor, almost entirely, Either they support vfr, or they do not.

All of my devices are relatively new with the oldest about 6 months. Would commercial DVD/Blu-ray movies/TV shows have constant or variable frame rate. If a person uses Same As Source frame rate and selects Constant but the source has a variable frame rate, could this cause a rift in the space-time continuum?

It's not uncommon for all kinds of sources to be mixed interlaced/progressive, so with interlacing detection + deinterlacing (which you want) you will likely have VFR throughout, for instance a combination of 24p and 29.97i.

Players handle VFR just fine. They do pretty much exactly what HandBrake would do if you forced CFR, but do it on playback, which takes up zero space.