Having done lots of events and sports I've always found I'd be on the move the whole time (so it would be impractical to carry around equipment). I've usually carried a small camera bag (about the size of a crossbody purse) and of course my camera. I don't think I could/would want to babysit equipment when I need to watch what's going on to not miss anything and make sure I got the shots I wanted/needed.
I've usually done sports and events where it's not possible to use a flash at floor/event level, so I've done it using existing light. I've known photographers in a pinch to use a business card or cut a piece of cardstock to tape on the flash as a diffuser to avoid harsh light/shadows.
This may be more for future reference but it's good to go early, look up at the ceiling/around the room and notice the lighting. I usually try to face toward the middle of the room, and avoid dark corners. Figure out good vantage points, notice what the background will look like. Find out when and where specific things will happen so you know where to be and when. There are also many spontaneous things that happen and it's necessary to be aware of what's going on and to be able to work quickly.
I learned to go for a 'clean' composition, and keep visual clutter out of the frame. The length lenses being discussed sound like about what I'd use (usually 50mm, 135mm, maybe a 90mm, it depends on the size of the venue/event).
Even if this is unpaid/uncompensated work (in my experience in local sports people are often 'paid' in free tickets) it would be best to have a contract or something in writing to specify usage, amount of time, etc. (such as for website/social media use, typically for one year with usage extended later if needed). Once you give someone photos they're out of your hands and you have no control over what happens, especially if there's nothing in writing. If the photos are for social media use only then photos provided would be sized appropriately for that purpose.
Just remember to breathe! take some deep breaths... I found it took a lot of practice to get good at it but it's always been a lot of fun.