I've been tubeless since 2014, touch wood I never needed to fix a flat on the road. That said, I never go out without some form of remedy. If you carry an inner tube, you need to make sure you can fit it inside the tyre, otherwise it is of no use. Sealant alone is probably useless, if the one in the tyre did not work, what would make some further sealant do the trick? The main issue is that if the sealant doesn't work, then the hole is probably too big to fit an inner tube and go... it will probably bulge and puncture again.
What I carry for long rides is a spare tyre + inner tube, a normal clincher and for questions of space I carry an "open tubular" type of clincher, which packs very flat and small, something like Vittoria Corsa is perfect.
Some use "worms" for unsealable punctures, there are videos online that show how to use them... they seem a bit fiddly to me, so I prefer the old fashioned inner tube.
Whatever you do, good luck for LEL!