Captain Cook himself said he was an invader.
This is from his diaries.
'We enter their ports without their daring to make opposition, we attempt to land in a peaceable manner, if this succeeds it's well, if not we land nevertheless and maintain the footing we thus got by the superiority of our fire arms, in what other light can they then at first look upon us but as invaders of their country. '
As this was then followed by documented frontier wars and massacres, utilising said firearms, it is true that the first fleet saw themselves as invaders.
This doesn't need to be a reflection of modern day Australia. Though it informs some context. You are allowed to love and celebrate our country. But you can't just ignore what brought it to what it is now. You can't completely disregard the uncomfortable parts, and create your own version of reality.
You may say 'we don't need to always focus on the negatives on Australia Day', which is an understandable sentiment, but again raises the question of why we choose to specifically celebrate the invasion.