From my experience, you should level up the event-exclusive weapon to its maximum level during the event. The thing is that you get an EXP multiplier during the event that an help you save resources, and you can use them elsewhere. Also, during the event, you have unique buffs to weapons that can help you to bring out that weapon's insane potential. Festering desire had an event boost, and people were speedrunning content in the Dragonspine region. It can be safely said that you need to take the most benefit of event-exclusive buffs so that you can optimize your gameplay. There are plenty of rewards during major events, and missing them might be a loss if you are a free-to-play player. Major events offer free resources to participants, weapons, and even characters. So, I highly encourage you to take some days off your schedule and try to complete these events.