I did some research on mice, and I thought I'd pass it on. First, though, let me suggest that you get some of this stuff. Use it to paint a symbol on each of your wireless mice that matches a symbol on each of your mice dongles. It will help keep you sane. You'll still find yourself constantly in possession of mice and dongles that do not match, but at least they will have these pretty little symbols you drew all over them.
There is some interesting and exciting stuff going on with mice.
Best but most expensive small mouse for general mobile use
The Logitech MX Anywhere 2 Wireless Mobile Mouse, Long Range Wireless Mouse is over fifty bucks, but it has some excellent features. It is small and portable and normative in shape and design. It works on any surface, is highly precise, nice to use, all that. It is a Laser tracking mouse. It has an internal rechargeable battery.
This mouse uses a small USB dongle or bluetgooth (Bluetooth Smart Ready). You can pair up to three different devices. It has hyper-speed scrolling.
The Most Magical of Mice: Flow technology
There are several mice in this category ranging across price. One of them is the Logitech MX Anywhere 2S Wireless Mouse with FLOW Cross-Computer Control and File Sharing for PC and Mac - 910-005132, which is close to 80 bucks, and is like the MX Anywhere 2, but has the additional magical capability of controlling multiple devices, including managing a cross-device clipboard. You pair the mouse up with each computer, then you tie it into the same local network both computers are on. Here's a video from Logitech:
This supposedly works on Linux, Macs and Windows.
I am suspicious of the whole ergonomic thing. Ergonomic, in mice and similar devices, seems to be "we fit your hand so well you will only move one or two muscles ever," which seems a bad idea. I think a mouse should require more movement and adjustment by the hand in order to Not cause repetitive motion syndrome. Note that this is entirely my non-expert opinion and I may be quite wrong.
Anyway, one of the top rated and coolest Ergonomic mice is probably the Anker 2.4G Wireless Vertical Ergonomic Optical Mouse which is extreme in its design and intended to minimize RSS. The same company makes a variety of products, and note, these are generally not expensive.
General all round mouse
The affordable Logitech M720 Triathalon Multi-Device Wireless Mouse pairs with multiple devices, has fancy buttons, has hyper fast scrolling capability, and uses a single AA battery. It uses bluetooth.
Glows in the dark
I have a keyboard that glows in the dark. Maybe I need the ASUS ROG Gladius II Aura Sync USB Wired Optical Ergonomic Gaming Mouse with DPI target button. This $100 computer critter is a high end gaming mouse, and note that the interface is a wire. Proof that new technology (in this case, wireless interface to mouse) is sometimes inferior, and the old technology gets you more.
The Logitech M330 Silent Plus Wireless Large Mouse is a large size mouse that makes no noise and is inexpensive (and wireless, but not bluetooth)
The super accruate, wired, Corsair Gaming M65 Pro RGB FPS Gaming Mouse, Backlit RGB LED, 12000 DPI, Optical is for gamers and has lots of buttons.
The mouse I need is probably the one I hope to find over at Goodwill; I need a plug in USB mouse to allow quick access to any computer any time without needing a dongle dangling off the back of something.
You may be suspicious of the whole ergonomic thing, but as someone who has had very painful hand problems for 20 years, I think rather differently. I finally got roller ball type mice which are extremely helpful. Since my right hand is the most painful, I put the unit on the left side and use my leftmost finfgers to control it. It is not necessary to grip the unit at all, saving my thumbs. Having worked for years at the Harvard School of Public Health occupational health department, and followed those faculty members who established the Work Envirioment Department at UMass Lowell, I was very well placed to keep track of ergonomics for years. I have been retired for some years, but still use a computer for writing and design.
Microsoft Starck mouse.
Mary, it sounds like you are doing exactly what I would have recommended. The main thing you did was to switch hands, which totally obviates the elimination of variables, of course. Having a loose grip is hugely important. If you look at the grabby full handed ergonomic mice being sold, I think you might agree that they are not built for a light touch with fingertip control.
As a harvard trained biological anthropologist (we have to study skeletal and muscular systems) I'm certainly not suggesting that the research is suspect. I'm suggesting that the use of "ergonomic" in the advertising of certain products is suspect.
MikeN, do you like that mouse? It tends to get very mixed reviews.
I second the observation that ergonomic mice (mouses?) are good stuff and not just hype. I use the Evoluent Vertical mouse (evoluent.com), which is similar to the one you suggested. It is significantly more comfortable on the long term than a regular mouse, and also comes in a left-handed variant (I alternate semiregularly between hands, which also helps). Basically, switching hands lets your "bad" hand rest while the other works; using an ergonomic mouse lets you go longer before you need to switch.
You missed another important thing when it comes to buying mice:
Don't simply order one online, no matter how good the reviews are. Make sure you actually get your hand on one, either at a store or a friend's.
This will let you find out that the "best but most expensive small mouse for general mobile use" would be a constant annoyance for you because the size and proportions of your hand will result in constant inadvertent button presses.
I did like the Starck mouse.
On the topic of Ergonomics and RSI - I started developing RSI from mouse use a few years ago, and eventually saw a specialist.
The point he stressed to me was the need for variety, and not simply use the same muscles fixed in the same tiny range of movement day in and day out.
His advice at the time was to buy 100 mice and alternate between them.
My solution was to buy an adjustable mouse and adjust its configuration daily.
Still doing so, and now on my 3rd MadCatz RAT7, and still pain-free.