Model and measure. There are a number of spreadsheets such as John Brandt's that work if your room is proportioned as a rectangle. Mine isn't and required additional calculations. For a normal rectangular room, you enter the dimensions and it will calculate room modes at various frequencies. Bottom end is the greatest challenge so it is interesting to read your comment about muddy bass.I will add a question here, how does one know they have a well treated room? I see the old clapping trick, listening for echoes and such but are there any tried and true methods. Identifying a real bad room seems fairly easy?
After modeling, as someone famous once said, trust but verify. REW is a free software that you use with a measuring microphone. Doesn't have to be particularly expensive. I use a Behringer ECM8000 which is $42 on Amazon. It is useful for finding more ideal mix and monitor position. It is again, another rabbit hole one can go down. Once you know how bad your space actually is, one can obsess about it. Figuring out what all the graphs and data means is another big learning curve.
I took about 6 months for my studio upgrade working out the treatment design with measuring and mocking up various absorbers. My next house when I retire won't have a studio but will have a listening room. Having a room where I can really hear every detail in the music I love is one my life's greatest pleasures.