Interested to hear impressions here on the rg35xx h. I have a miyoo mini, but it's just too small and awkward to hold really for any length of time.
I've had mine for a little over a month now and it's been great. Size hasn't been an issue at all for me with it being around the size of a DS Lite. Buttons and dpad are good, though I found I needed to remap l and r to the "triggers" to make it more comfortable for me. I wouldn't call it the most comfortable device in the world but I've done a few multi hour sessions on the thing without any discomfort. The only big issue are the sticks which feel terrible due to being essentially locked to 8 directions with a massive deadzone. You can somewhat alleviate this by changing the deadzone and sensitivity settings in emulators but it doesn't do a whole lot. Neither does replacing the sticks. Apparently it's a software bug though so it might be fixed in the future hopefully. Sleep mode doesn't really exist either I guess. You can technically put the 35XXH to "sleep" but it just shuts off the screen essentially. It hasn't really bothered me though since I'll generally just fully switch it off when I'm not using it.
I've been using Batocera as my OS the whole time, never actually even used the base os, but it's overall a pretty smooth experience. The default settings are generally good, though it uses the XBOX button layout by default for some reason. This isn't too much of a hassle to change but it'll need to be done for every emulator and AFAIK can't be changed for the standalone N64 ones and needs to be done every boot for Drastic. Having such a nice looking interface makes it worth it though and scraping boxart and videos is super easy once you have a screenscraper account set up. I'm also a big fan of being able to add background music to the system. There's no dedicated audio player app but I've spent a decent amount of time just vibing to music while watching the screensaver go by.
The only other real annoyance I've had with Batocera is configuration. By default you're expected to just do all your configuration through Batocera itself, and that config gets loaded every time you launch a game. Unfortunately the options you're given by the OS to configure your emulators are extremely limited, with stuff like controls being completely absent for example. You can get around this in RetroArch using per game/core/directory overrides, and for standalone PPSSPP and Flycast by doing per game configs, but for the standalone N64 emulators and Drastic you don't have the option hence the aforementioned button mapping issues.
Performance is solid for everything up to PS1 for consoles and GBA for handhelds. You've got enough headroom for upping the resolution on PS1 games even. The only issue I've had with the really retro systems is some minor slowdown on Virtual Boy (like one or two frames, nothing major). DS, Dreamcast, N64, and PSP are nice bonus to have but aren't perfect. I haven't had many problems running any DS or Dreamcast games but a lot of games will need frameskip enabled, especially if you're running upscaled DS games. The lack of a touch screen limits you on the DS games you can play as well, with the sticks not serving too well in emulating the stylus. N64 is a little worse off, still generally fairly good but you'll need to fiddle around with different emulators depending on the game, and stuff like Perfect Dark is a complete non-starter. Dreamcast, PSP and N64 also suffer pretty heavily due to the stick deadzones as well.
PSP is probably the worst performing of the systems you can emulate here but you can get a surprising number of games into a playable state despite that. Slower paced stuff like rpgs tend to run pretty well out of the box, and any 2D games hasn't been an issue, but with a bit of fiddling I even managed to get one of the GTA games to a mostly playable state. It'll really depend on how much you can stand things like under clocking and frameskip though.