Ugh, hashtag optimizing is soul-sucking, but you do not need to hand your photos to some random web service to get tags. Run a tiny image-caption/tagging model locally and convert the results into hashtags. My go-to is clip-interrogator, it runs on your own machine, spits out concise keywords and prompt-like descriptions, and is literally made to extract useful phrases from images. Github: https://github.com/pharmapsychotic/clip-interrogator
If you want something more generic, use BLIP (Salesforce) image captioning via Hugging Face, e.g. the Salesforce/blip-image-captioning models. Install with pip, run the model to get a caption, then use a simple POS filter or spaCy to pull nouns/adjectives and prefix them with #. That workflow is trivial to script and keeps everything local. Models will run on CPU but are far faster with a GPU.
If you absolutely need Android, use Termux and a lightweight model or run the model on a tiny home server and call it from your phone, do not upload to third-party servers. For anime art, DeepDanbooru is the standard local tagger. For everything else, clip-interrogator + a tiny post-processing script is your best bet.
Honestly, stop treating hashtags like SEO black magic, generate sensible descriptive tags locally, and move on with your life. If you want, I can paste a minimal Python snippet that takes an image, runs BLIP or clip-interrogator, and outputs a ready-to-paste list of hashtags. Which model do you want to try, BLIP or clip-interrogator?
Stop freaking out, you can do this. Don’t try to build a full server farm on day one. Start small, get something that actually works, then iterate.
Practical path: plug an external HDD into your Linux Mint box and install Syncthing on both the Pixel 8 and the PC. Syncthing is dead simple for backups, it syncs your phone photos to the PC with no cloud, no port forwarding, and it Just Works. Install with apt on Mint, install the Android app on the Pixel, share the camera/DCIM folder, accept the device link. You’ll have automatic backups within an hour and you’ll actually learn how files move around.
When you’re comfortable, add a second copy or offsite backup (cheap VPS, friend’s house, or a rotating drive). If you want a web gallery, user accounts, or calendar/email too, then move to Nextcloud or a small NAS OS like OpenMediaVault or TrueNAS SCALE on a dedicated box or a Raspberry Pi. Use Docker if you want portability, and always put HTTPS and a firewall in front if you expose anything to the internet.
Bottom line, stop reading dozens of guides. Do Syncthing + external drive today, then upgrade. You’ll learn a lot faster by doing than by overplanning.