I tend to agree with what has already been said. It is unlikely that for $500 you are going to find a mixer (or any one or collection of outboard boxes) that will magically add "warmth" or spacious to a mix. Warmth is at best subjective and generally a product of different types of distortion (typically favoring, extenuating even ordered harmonics, very roughly speaking . . . & while, particularly with a cassette recording process & cassette mixdown, a stand alone aural exciter (or running a digital mix through a final tape pass) could marginally improve a subjective impression of warmth it seldom succeeds in effectively mimicking the analog ambient 'grail' for which one was searching . . . so typically 'warmth' on the cheap is a convoluted rabbit hole once entered difficult to exit and ultimately not cheap . . . which isn't to say picking up an old 2nd hand 'exciter' is a bad idea.)
Spacious mixes on the other hand start out, in analog realm, with space. If you are tracking one instrument at a time in small rooms (with parallel surfaces) you typically start with an unified cramped sense of sonic space. Correcting for this in post (even when tracks are stored digitally) is more difficult (not theoretically harder but typically more time consuming) in analog OTB vs digital ITB modes. And it is inevitable that each back to analog pass will cost you in worsened S/N. Sense of space is not purely timing. It also stems from how human sense of hearing responds to different frequencies emanating from similar distances (to other frequencies). How different frequencies respond to ambient obstruction (the stuff that creates 3D reverb), how all the different frequencies (fundamentals & overtones) of all the different voices interact with human hearing. Constructing a sense of space (from cramped originals) is not solely a function of panning, but also EQ, level & dynamics. Very roughly speaking, particularly with entry level gear, achieving a 'spacious' mix is easier if a rough mix is executed on the way to tape. This presents something of a paradox because editing is almost always more satisfactory with files that have the fewest artifacts imposed by the recording process. This poses another issue for cramped spaces because spacious mixes are more readily achieved in response to waves in 3D space, i.e. speakers vs. headphones.
It is possible that a slightly different inexpensive compact mixer might subtly improve some variables. I tend to be slightly happier with fader response and EQ on A&H compact desks then with Mackie, my gut reaction is that Mackie onyx pres are a toss up with similar vintage A&H (again for example) but it is unlikely on OPs budget that any mixer change is going to produce dramatic improvement . . . unless there is something significantly wrong with their current HrdWr.
What is likely to produce the most significant improvement cost/benefit per dollars spent, is to improve the mix/monitor environment. Depending on what monitors OP has, $500, in DIY, mode should, just, provide for significant improvement. Absorption, reflection, & diffusion, within a very specific space have to be addressed . . . and typically you are addressing one small location within the mixing environment. If you do not accurately hear what has been recorded no matter how vintage and/or pricey the gear mixes will seldom be satisfactory.
good luck