At 60%, the battery will drop to 45% percent before recharging to the specified percentage for coulomb counter drift compensation.
Most lithium UPS avoid this by using LiFePO4.
Moto G04 with 128GB Storage (UFS - 400MB/s read/write), 4GB RAM and 2TB capable SDcard UHS-I slot is just 90 USD.
ZRAM is enabled by default at 2GB and 2GB storage swap, it's adjustable under "RAM boost" setting.
Disabling the app background killer is one toggle away for Termux.
No root required.
Wi-Fi does around 350Mbit with no buffer bloat, and supports simulatenous softAP and station mode with a dedicated DHCP by enabling hotspot without mobile data. This is useful for out of band management, e.g if Wi-Fi AP goes down, there is a small hiccup though.
You can combine Speedify and Tailscale (for NAT) to have 99% uptime with seamless failover since there is a mobile modem in there. ;)
The Type-C port is dual role and will charge from a powered hub in host mode. However connected devices power is disrupted for a split second if the charger is removed, so the phone isn't really a UPS for external peripherals unless you don't mind the brief reconnection.
I do hope a Pi A+ or something similar and popular exist in the future at a reasonable price, considering that we have phones that are cheaper than the Pis while missing GPIO but adding camera, speaker, screen, case, charger, LTE, and a headphone jack which was also removed on the Pi 5 (USB DACs aren't consistent with pipewire).
Mini PCs vary and they aren't really portable, hard to setup headless, especially projects utilizing an easy DIY approach like the Pi-hole or kit/pre-flashed SDcards.
For comments comparing a new Pi with used PC: used Pis exist and are a lot cheaper after shortage.