>Do you suggest that I should invest a monthly sum into these funds (maybe 60-120 dollars a month)?

"As much as you can as soon as you can."

Please also see the /r/personalfinance Prime Directive: https://reddit.com/r/personalfinance/w/commontopics

>I am using Fidelity, so I'm pretty sure I can do fractional ETFs.

Correct, and Fidelity also has excellent index mutual funds to consider (though the Zero funds especially should NOT be held in taxable accounts due to inability to be moved to any other brokerage).