At what point are you shooting enough film (b&w) to make getting into film processing worthwhile?  

It doesn't look like the chemicals are outrageous $$,  unless they go off from sitting around for too long.

On the other hand is mail order processing a firewall to keep this hobby in check?

When I started shooting in film a few years ago, I immediately got into home developing.  I love it.  The chemicals do go bad over time so I've gotten into a routine where I shoot a bunch of film then I develop as much as I can in a weekend or so.  

Bear in mind, different developers have different aspects of degrading.  D76 is what I use for black and white film developing.  It'll last a few months and you can refresh and add back into the mixture.  It will weaken and your images will come out with less contrast when that happens.

C41 is the most common color negative process used today.  It's shelf life is a couple of months and it has a very finite number of uses before it loses it's punch.  

E6 is for color reversal or slide film.  It's a more toxic process but if you master it, you can get some amazing results with films like Fujifilm Provia and Velvia.  Both offer amazing results but each is specific towards different color gradients.  Velvia works better in cooler color temperature images while Provia seems warmer.

Film developing may seem daunting at first but it's really not.  It'll take some time to tweak your skills and technique to come up with the results you want.  

For the record, I shoot 35mm, 6x4.5, 6x6cm medium format, and 4x5in large format.  It's a horrible, horrible addiction.

I stumbled into a 4x5 and it looks like I'll need to do it myself if I'm going to use it much. If I go that far I might as well do some 35mm too.

Preliminary research indicates I can let rodinol sit around forever, and sodium thiosulfate  crystals to be mixed as needed for fixer probably likewise if I keep it sealed.

Rodinol is good but I think it breaks down as well over time.  Hell, try it out and see how it holds up.  Let me know how it works for you.  I may switch.

I haven't been in a dark room in over 35 years, but back then I did a lot of 35mm B&W.

Processing your own allows you to adjust your exposure to the paper, not sure what the process is called anymore, its literally been that long, but darkroom work definitely takes your personal involvement to a new level...sometimes I miss those days.

Normally, I would amass about ten rolls before I went in, just to make the outlay of time and chemicals worth it.

Also, I kept a small notebook with records of lighting conditions and camera settings for each roll and pics when I could.

I just got a Yashica Electro 35GX hot off the plane from Japan yesterday. I've got some Ilford HP5 Plus I plan on putting to use over the weekend. Would this be good to get started developing negatives? I don't really have a space to have a full darkroom to do prints so I'm just looking to develop/scan the negatives.

https://www.amazon.com/Paterson-Ilford-Processing-Developing-Bundle/dp/B09BZRRRCT

Maybe, that kit checks a lot of boxes, but it has some extra stuff and is short a few things too.  If it were me I'd shoot a test roll and send it to a lab to confirm the new camera works before investing more.