Given the timeframe of 3 weeks I’d say your second approach is going to give you more bang for your buck.
It’s also likely that there are a whole bunch of things other than just fitness that you can improve to give you a better chance of staying with the bunch - cornering, positioning, braking (or rather not braking…) so that you burn less matches.
And last thing is that if you can’t currently stay with the bunch then your goal may be unrealistic. I.e. If you do manage to stay with the pack then it may take everything you’ve got to do so, and you won’t have much sprint left at the end. No harm in having stretch goals as long as you don’t get too dispirited at not achieving them.