Some of the old LG phones (the V series) were known and sold for the quality of their audio. If you can find one of those on eBay it should be pretty cheap.
I use a Sony NW-A55 walkman. It is one of the latest models of digital audio players that doesn't use Android. It has a microsd slot, supports flac, bluetooth, and no internet connectivity. The japanese models are still available new on ebay, you just have to change the language on the player to english.
You should look into Rockbox. It's an open source alternative firmware for a variety of portable media players that adds features such as support for more file formats (and it runs Doom). As for which supported players I'd recommend then maybe a flash modded iPod or an AIGO Eros Q or K (also sold as the HIFI WALKER H2, Surfans F20, or AGPTek H3).
Yes… in the cassette players that work with those adapters. Annoyingly, the old stereo we have set up at work in one shop doesn't work with those casette adapters or the Mixxtape.
Also, if you use it in a tape deck, it doesn't use the spools as inputs. You just set it playing and pop it in. Similar to my old Digisette Duo Aria.
I will admit, I have rarely used the tape deck function, but it has been useful on occasion.
Rockbox can be installed to mitigate that. And the hard drive can be swapped for bigger flash storage, which also has the positive effect of leaving more room for extra battery.
Either dig up an old iPod (flash-modding would be a good idea, as 300+GB iPods are rarer) and put Rockbox on it for FLAC support, or dig up an old phone or something and get a decent USB-C adapter/DAC.
I am currently rocking a Shanling M2X which I bought secondhand for €75,- from the local classified ad websites.
Officially supports up the 512Gb but according some reviews it can do up to 2Tb microSD cards ( https://headfonics.com/shanling-m2x-review/ ). I currently have a music collection of about the same size. The Shanling M2X can drive all the IEMs I throw at it, has enough file system support to read just about anything and has WiFi and Bluetooth. Although the WiFi can only be used to wirelessly sync files (which can be a pro but also a con, depending on how you look at it).
However if you don't feel like going secondhand, Shanling offers a whole bunch of devices meant for offline playback of local files.
It's a great player, over 120DB SNR! and DSD if you think you need it.
Codec wise it supports most everything except Opus but is now discontinued (FiiO only sells players for the uber-rich now - entry level is US$499.99 for the M11S)
If you can get one used, they're a great player.
I've heard good things about the HiBy Digital M300 - but i've never tried one.