I have a blog that's nearing 8 years old now and yes it got 300,000 monthly unique visitors too at the 5 year mark.
Once it was online for 5 years and traffic picked up due to all those blog posts I typed up over those years the traffic started increasing.
I was making between $1,500.00 to $2,500.00 from from Adsense on my blog and now that my blog is almost 8 year old and is bringing in 1 million visitors per month I make between $7,000.00 to $10,000.00 per month from my blog depending on how much the Adsense ads pay and what my CTR "Click Through Rate is".
So yes you should make some money from Adsense if they approve your blog.
300,000 monthly unique visitors is certainly good enough to make a decent side income from.
Keep working on that blog and overtime you'll make more than your day job and can quit the job eventually like I did to stay at home and do the things I love instead.