I have a very high-tech portable MP3 player, as you can see from the attached photo. SWMBO does not want me using it any more on my electric trike because it is not compatible with a helmet.

This matter came to a head (pun intended) following an unfortunate altercation with a four-inch thick low-hanging branch on the local bike path. I rode my trike to the hospital ER to be stitched up, then rode home and got my hiking saw, rode back and cut that fscking branch off, then rode home. The doctor at the hospital told me that if the branch had been one centimeter lower I would have lost the eye. FWIW, the doctor's fee was $500 Mn pesos; the hospital fee an additional $600. 20 pesos per dollar conversion rate... you do the math. Gotta love Mexico!

So, SWMBO has promised to buy me a very good Bluetooth headset that will fit under my helmet. She has already chosen the headset... but herein lies the problem. I do not own any kind of Bluetooth compatible portable MP3 player. I'm sure there are plenty of them out there, but my absolute deal-killer priority is that it ABSOLUTELY MUST be compatible with audio books. This means fast forward/rewind; track advance/back; pause/start; and it MUST remember the exact position when powering down and starting back up.

That last requirement is not so easy to come by. I would pay extra for water resistance for my kayaking, but my present technologically advanced system is still useful for that. I guess either a stand-alone portable player, or perhaps there is a cell phone app that would meet my audio book requirements and pair my Samsung phone to the Bluetooth headphones while on the trike. I won't take the cell phone on the kayak.

Recommendations?

tanstaafl.

You could also use the existing MP3 player with a bluetooth transmitter dongle. I have successfully used the following bluetooth dongles, but, plugged into an airplane armrest instead of an MP3 player:

https://www.amazon.com/gp/product/B082G2Z1WS
https://www.amazon.com/Twelve-South-Transmitter-Headphones-Airplanes/dp/B0BB8FVKFC

There are many more like these. I do not specifically recommend any specific brand over any other. They are plentiful and varied; perhaps searching around for things in this category, you can find one in a form factor that meets your needs, based on their features. For instance, the "Airfly" one that I linked above has an alternative "duo" version available which lets you pair two sets of bluetooth earphones at a time, for couples listening.

The only drawback is that you must recharge both the dongle and the MP3 player. If you've ever had the MP3 player run out on you at a bad time, then you're basically doubling your chances that that'll happen to you.

If you don't want to use a separate dongle, then, as you already mentioned, almost any smartphone (even the cheapest ones, even if you don't buy a cellular plan for it) will work as a perfectly serviceable MP3 player with bluetooth built in. There are multiple available audiobook apps for them which you could experiment with.

I haven't found an iOS player that works reliably for audiobooks (forwarding, remembering position)

The iPod Shuffle (shown in the first picture) works very well. But as you pointed out, the great majority of players, iOS or otherwise, just don't.

tanstaafl.

Perhaps try other audiobook player apps, on both Android and IOS?

https://www.digitaltrends.com/mobile/best-audiobook-apps/

Frame challenge: get a helmet with built-in wired headphones?

They used to exist for winter sports. I don't know whether winter sports helmets are suitable for cycling, and I don't know whether you can still get the wired ones, but it's gotta be worth a look.

We use them for an entirely different outdoor sport, but I will mention the Cardo devices.

We use Cardo PackTalk Bold but any of the other models that cost less should also provide Bluetooth audio link to smartphone and work with whatever audiobook app one prefers. The Cardo DMC mesh radio tech is used to communicate between people at a distance. Bluetooth is always used to connect to your own phone.

The Cardo radio modules are an add-on to whatever helmet you have. Most of us use a full-face helmet with clear windscreen (similar to motorcycle/motocross). The flat disc speakers are inserted into the ear areas inside the helmet, and plug into the Cardo radio module.

Cardo also sells an over-the-ears headphone kit, to which the Cardo radio unit clips. No helmet.

Cardo is probably overkill for what was requested for hiking, but I suspect similar products exist for bicycling, with or without the ability to talk to other riders at distance.