Im in the same boat here too. Used to float fish quite a bit. I dusted off the float rod from the garage and noticed that my main line has been on my reel for almost 7 years.
I only have about 30 yards of line.
If you are not concerned about the look and colour of your backing , I would suggest using 10 pound red wolf ( the cheap stuff from crappy tire ) Cost about $ 4 for a spool of 1000 yards. I use this stuff for backing on all my reels. Very cost effective.
As for main line, I used to use Super Silver thread 10 pound. Im looking for options to re spool my float reel as well. I should be good for another 7 years.
Then I have to purchase hooks , mesh, floats, split shot.
Thinking I might have to hit the crick for some after work drifts