Hi and welcome to the forum:
To answer your question: The aperture on the EF 24-105L f/4 is a constant, while the RF 24-240 f/4-6 is variable - as indicated. As you use longer focal lengths on that lens the aperture will get darker.
The EOS R body, being full-frame, and having fewer but larger pixels should exhibit considerably less noise and better low-light performance than the R7, so I would use the R7 purely as an emergency backup. Where possible shoot available light, and that is where the EF 24-105 f/4 will work well.
If your Sigma lens is the Sigma 18-300mm f/3.5-6.3 DC Macro OS HSM -C, it is designed for a crop-sensor body - so should remain on the R7 rather than the R body: while it will physically attach to the full-frame R body via the adapter, the sensor capacity will be reduced from 26MP to about 10MP because the image projected will still be the size of an APS-C sensor, thus reducing the area recorded by the FF sensor by a factor of 2.65. The Field of View of that lens is also the equivalent to that rendered by a 28.8-480mm lens on a Full-frame body, so do you need those characteristics? I would question if this is going to be useful with a flash, which will have limited ability in terms of distance, which I will get to momentarily.
What do you know about the conference venue? Environments can vary dramatically from bright outdoor or well-lit rooms to rather dark places with a lot of projector talks. If you are know where it will be held, do research into that, visit the place and find out about the program and any restrictions on using flash - some don't allow that. Having flash firing can be very disconcerting to attendees and presenters.
You also need to know how close you can get to the subjects: are you shooting social events where you mingle, or in a hall with the speaker some distance from you? These all have significance to focal length and lighting. If you are doing this officially, try to get a seat reserved for you in a strategic spot that YOU have figured out is best - don't rely on others.
You have not indicated what model flash you will use, and those vary dramatically in output levels. Ideally, you want a flash that is fully compatible with the Canon R bodies so it works as transparently as possible. The easiest method is to use the ETTL metering, but honestly if you have not used a flash before, you need to get some experience with it before you commit to doing something for a client - paid or not. A small flash will not do much for you at longer distances, and focal lengths. So you would have to consider something with a bit of power if you are not that close.
Frankly, it would be ideal if you can shoot with available light, using an open aperture and reasonably high ISO. I did a conference using the EF 24-105f/4 predominantly, and never had a need for a flash.
If you need to get group shots, take them outside, preferably on a set of steps, avoid harsh light as people wear sunglasses or squint, so light shade is preferable. Tell them if they can see the camera it can see them - take several as there are always people blinking, looking away or yawning.
In the venue, keep your verticals vertical - you aren't looking for drama that comes with tilt - it is disconcerting. Move so you can get specific sections of the audience. This was an overall image showing the environment.
Shoot from different angles to catch different perspectives of the room and attendees
Be mindful of the benefits of being close to the speakers, and try to have them placed in front of their displays
You will want shots of individuals, so again being close helps a lot, don't be afraid to move around, especially if you are the official photographer.
I hope this helps, and others will very likely have valuable guidance
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