It sounds like you're conflicted in several areas...
The first is if you think you should upgrade your mic/pre, the second is how to mount your current mic.
I have no experience with the Blue Yeti mic. I'm assuming it's a USB model, as you mentioned being limited in your funds to get into an actual XLR pre/interface...
But, you must be less than satisfied with it in some way, because you've posted here for advice.
There are several different ways to mic a piano, some of those includes multiple mics, and obviously you can't do these with just one mic.
Mic'ing a piano with one mic isn't impossible, but generally very high quality mics are used when doing it. I'm not sure the Yeti falls into that category.
There are several things that determine the sound of a recorded track.... The first is the instrument, (and player) the others, in no particular order but all just as crucial, are the room the instrument is in, the mic(s) used, the mic preamp (s), and the ability of an engineer to choose the best placement for the instrument in relation to the room, and to place mics where they sound the best.
I can't say for sure, because I'm not familiar with the mic you are using, nor am I familiar with the instrument you have or the acoustics of the room you are recording in, but I'll wager a guess and say you're probably not gonna get the sonics of a Bruce Hornsby or Billy Joel recording.
There are many good reasons to get into a multi input channel pre and 3-4 good microphones to choose from... Not the least of which is the ability to do multi mic'ing on things like pianos and drums, or having the ability to record a piano and vocal at the same time...

As far as the boom arm you mentioned, instead of trying to rig something that might fail,, damage a mic, or trash what might be a keeper take, why not just get a new mic stand with a boom built in?

As an addendum to the above, it really all comes down to your expectations.
There's a reason that great sounding commercial recordings with real pianos sound the way they do...because great instruments and mics/mic pres are being used. I think it's a safe bet to suggest that when Elton goes into the studio to record, they aren't using a Blue Yeti USB mic anywhere in the signal chain. ;)
Technically, you can use "any" mic to get sound from point A to B. But how it sounds will be determined by the weakest link in the recording chain...that chain will only ever have a sonic quality equal to the link with the least amount of fidelity and sonic integrity. It could be the mics being used, the mic preamps, the AD conversion, the room, or the instrument itself.
Can you post a sample of what you've recorded using your current rig?
At the bottom of your posting window, located just to the right of "post reply", is the "upload a file" button. Click on this, it will bring up your computer directory, select the file you want to upload, and it will put your audio file here for us to listen to.
RO's file upload will support MP3's up to 320kbps, with a file size limit of around 15 Meg.
Be patient, the time it takes to upload is determined by your own ISP and its UL rate. Generally, files are uploaded within 5 minutes.
Let us hear what you have thus far. Perhaps we can make recommendations.
;)
-d.

Hi Donny! Thanks for your detailed feedback.

The room that has the piano is very, very bright (something I need to fix with some sound absorption panels/rugs/etc anyway... working on that :p ), so that obviously accounts for some of the hollowness/brightness/lack of the warmth level that I want). I also haven't really figured out what's making the background humming yet (-_-), so my older recordings aren't the cleanest (also working on that, but I haven't been recording much lately... bit discouraged). Anyway, my piano itself isn't extremely bright, as I prefer the warmer sound, but the room changes it quite a bit.

Also, the yeti (which is USB, you're right) is really heavy, so there are only a few mounts that can take the weight, and I haven't found one that's floor mounted yet (part of the problem). The Rode arm is supposedly capable, but again, I don't know if I should buy it or save it to put towards a bigger purchase. (All I know is I really need a shockmount and arm, because the vibrations are driving me nuts :D )

As for placement, I've found a couple of videos that experiment with the placement and settings, so I'm hoping to try those and see how it works, but I'm still going to need some sort of extra equipment for a mic, and I don't really want to spend money on something that I might stop using soon.

The only sample that I have with my grand (which I only recently purchased) also has some string stuff that I added by messing around on my DAW, so you'll have to listen through that a bit (sorry). I'm still not very familiar with how to properly add compression/EQ/etc (I know a bit about them, but not how to actually apply them to my stuff), AND the mic placement sorta sucks after listening to some other samples, so try not to cringe. (Still probably going to post it on my YT channel in a few weeks soon though, which I just started... at this point I'm just trying to get material out on it, but I really want to improve the quality of at least the sound). I tend to be a perfectionist, though, so idk... Anyway, here 'tis (you guys get to hear unreleased stuff... whoopie... -_- )! :D It's my version of Evanescence's My Heart Is Broken.

-S

[MEDIA=audio]https://recording.o…

Attached files Cover - My Heart is Broken.mp3 (4.9 MB) 

Also, forgot to mention. I found out later that the Yeti is very, very sensitive, so the gain usually needs to be turned way down (almost all the way) and most of the fiddling done with the computer input levels. Clearly I did NOT do that on the sample I posted... :p The mic itself has all its settings on it since it obviously doesn't use an interface. I'm just lucky I didn't get painful amounts of clipping. And btw, the vocals and piano are recorded separately.