The Bernina Bernette 35 has gotten pretty good reviews- it's mechanical, and has a threader feature:
https://duckduckgo.com/?q=Bernette+3...=v314-1&ia=web
it's available lots of places, including online
this one is (barely) under 300 under "buying options"
https://www.amazon.com/Bernette-Swis.../dp/B06XHBSRGN
of course, online doesn't come with dealer perks....
back in 2018 I was in a quilting group that had a sewing/ classroom available, and there were 4 Bernettes available to use. Most of the ladies liked them, except for the somewhat small throat space. Can't remember which Bernettes (33? 35?) they were though.
You could probably use accessories/ feet you already have for your Big Bernie?