Keep the machine, just use PTFE thread.
It requires practice, a lot of tension and a side feed off the spool preferably with a way to allow the spool to rotate easily. You need a guide so the thread will feed to the machine so it thinks it's coming off the machine's normal spool location (more or less).
Being monofilament, it will make hockles and give you grief if you don't follow the above advise ! It's also relatively expensive, but once you get past the above cons, it's SO WORTH THE EFFORT !
It is of small diameter, but is extremely strong and it never wears out due to air pollution, grit, ozone or UV. You read all that correctly.
I have previously posted about this wonderful stuff and repairing my multihull
mats every couple of years with standard sail thread. What a pain.
Then I used Tenara (GoreTex) PTFE thread. That was about ten years ago and it's still as strong and new looking as if I had sewed it yesterday.
We are currently in the process of getting replacement mats from multinullnets.com.
All their sewing uses this wonderful stuff.
The original 1995 mats material is finally failing but all of my ten year old stitching is 100% perfect.
If you DO replace your machine, imho, needle and machine thread diameter limitations are no longer a real problem.
Look more at usable throat size.
There is a Janome large throat with needle up/down which might fill the bill.
Metal construction, decent quality parts
and the plastic gears have never shown any problems in my shop. It will wind
a bobbin as you sew. No zigzag though. Otherwise a great machine.
If you need z z and a good walking foot, all the above recommendations about the thread still apply.
Imho, if the Sailrite
isn't a good fit, you indeed are searching for a Unicorn.
good stuff about correct needles.
Your machine may benefit from a good tech's touch.
Have the tech crank up the presser foot tension and be sure the feed dogs
are set high enough, and not worn smooth. They should have rather sharp edges to the traction surfaces. Easily replaceable.
Steve B. Certified Bernina Sewing Machine
S & J Sewing Machine Services