You pay a lot to have everything packaged in a nice neat box like a Bluetti. Inside something that costs a few hundred dollars, a pretty small $30-$40 battery frequently resides.
One of these:
https://www.jaycar.com.au/12v-7-2ah-...m-ufb/p/SB2486
With one of these:
https://www.jaycar.com.au/cigarette-...clips/p/PP2007
And a charger:
https://www.jaycar.com.au/6-12vdc-1-...arger/p/MB3900
This charger is compatible with all battery types.
I have some very old 12v 7AH SLA batteries that have been recharged more times than I can count. They still run my scope (EM200 - an EQ6 clone) for more than half of a night. If I don't intend to do an all-nighter, I often run my scope off these rather than using my big 120AH Li Ion.
If your power consumption requirements increase later, eg you add a cooled astrocamera, dew heaters, you can either buy extra batteries and put them in parallel or (better) you can buy a different bigger battery and the charger will work with a wide range of battery capacities and types.