You can use pre-built presets iZotope Ozone 8 or 5 or 7, whichever version you prefer. It should enhance the quality and it will give you compression as well in case you don't want to dive into the details of mastering engineering. I like all your tracks!
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Freelance mastering is a totally oversaturated market. It will be super tough to make anything more than some extra money on the side (maybe) until and if you can get into the higher end, which is a comprised of very seasoned and highly skilled pros who aren't going anywhere. Many people are offering mastering services for free, just to learn their chops, and many new musicians and bands are attempting it themselves with all-in-one plugins, like Izotope Ozone, for example:
https://www.izotope.com/en/products/master-and-deliver/ozone.html
My suggestion would be to get it and get used to it. But, in my opinion, whatever tool(s) you use, the results will be 90% or more dependant on your ears, your sound. That said, there are hundreds of tutorials on YouTube; just use an appropriate search term, and Bob's your uncle.
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Best first bundle is [iZotope Ozone](https://www.izotope.com/en/products/master-and-deliver/ozone.html] If you can't pay for it outright you can rent to own it on Splice here Nothing else is as comprehensive or highish quality as this to start. Was my first one as well and def a good investment. Even though I have much better stuff now I still use this on occasion for certain things.
Loosely, 80% of getting solid mix level occurs in the mix process itself. The rest can be optimized through mastering.
There is compromise involved in retaining a nice dynamic range while having a high average mix level.
Leaving headroom on your individual tracks is fine, but leaving it on the master is not necessary. That said, you have to be very careful that there is no clipping at all on the master. Often the suggestion is made to leave headroom on the master to ensure that spurious transients do not sneak their way through without you noticing.
Common mix mistakes are skewed distribution of frequencies in the mix itself. Usually too much low end that when compressed/limited without adept skill end up squashing the whole mix. This is very often the result of inadequate monitoring equipment, lack of judicious use of reference tracks, and general bass hunger.
Try not to overload your mix with very low frequencies and pay careful attention to mid-bass frequencies which can delivery solid useable power.
One technique that you can try is to put a compressor on your master bus while you work, with the threshold set at -3dB, and reducing the level by approx. 3dB with no clipping on the master. Then turn it off, and without moving the master fader, adjust your mix tracks so that there is no clipping on the master and your peak level reaches 0dB. Again, pay attention to the tracks that may have dense low frequencies.
That should put you in good shape for mastering.
Generally, with a good balanced mix, having someone else to the mastering is preferable, but if that is not possible, Izotope Ozone does a great job.
https://www.izotope.com/en/products/master-and-deliver/ozone.html
Beyond peak limiting, proper EQ/compression needs to be applied for the platform you are intending for playback.
Go here: https://www.izotope.com/en/products/master-and-deliver/ozone.html
If you move to a condenser mic it will probably capture more of the room and you might need "soundproofing", but with a dynamic mic I doubt you'll see much benefit in adding sound conditioning (btw proofing is when you want to block audio in/out).
I'd say that an easy way to improve your sound would be to get a better compressor or mastering plugin. MJUC is a great compressor with excellent price/quality ratio. Ozone is an industry standard in mastering although you'd probably need someone to help you set it for you. Then get a better mic. Then get a better DA (the thing that converts analog audio to digital). Then finally room acoustics.
I highly recommend this program. You can use it as a standalone app, so you don't need a DAW. It'll do wonders. It'll give you the big sounds you're going for, giving you control over pretty much all the frequencies. Not hard to learn.
https://www.izotope.com/en/products/master-and-deliver/ozone.html