in terms of free software this mastering compressor / limiter has been recommended to me by a few people: https://www.tokyodawn.net/tdr-kotelnikov/
i find it helps to think of mixing plugins in terms of transparent v character - the transparent ones solve problems i might have, and then the character ones take it in the direction i want it to go in. the key skill is identifying the problem and then choosing the right tool to solve it - is there too much information in one frequency range, is the mix too dull or too bright, is it boxy sounding, is it boomy, etc. it helps to A/B a lot with professionally mastered recordings and to do so on different systems (headphones, studio monitors, etc).
generally speaking, getting a professional sounding (loud, clear, warm, punchy, shiny) mix isn’t about one solution, but a lot of little things that together make a difference.