"Had a look to see if there were any PCB makers here. Found one, but it seems their prices start at $86 for one board"

Where did you look?
Try JLCPCB.com, it'll cost you $2 + about $4 shipping for 5 boards under 100mm x 100mm. It'll take about 15 days to arrive.
It seems to me you are trying to re-invent the wheel here. An on board glow system doesn't need to be cutting edge to deliver the maximum power to the plug that it can withstand, it only needs to keep the element warm. We have used nicad batteries for years to deliver 1.2V and that is ample for an on-board glow system. If there is no other power regulation to be done a simple Rx controlled switch will do the job (or a microswitch on the throttle servo for the luddites), they are cheap as chips to buy and simple to make if DiY is your thing (I have a dusty old draw full of them).
The RCEXL driver is also excellent if you must use a lipo.
In my experience aliexpress is no more a haven for scammers than ebay, amazon, etsy, banggood, temu or any other on-line selling agency (ok.. perhaps banggood isn't an agency but they are still capable of ripping you off).
The way things are going you might well not have the option of buying from China soon anyway.

You only ever need two tools....WD40 and duct tape.
If it doesn't move when it should use the WD40 and if it moves and it shouldn't use the tape.