Get an Etude book - something modern. You need muscle and eye memory, so you can recognize most notation instantly and know how it sounds. Ear + muscle + eye work as one unit. This is the key to plain sight reading. You really want to get to a space where you are reading one bar ahead. Is that your goal, like a n orchestra player or for hire that can do cold reads? Or do you want to just read and study new material and memorize it and glance once in a while to make sure you got it right. The former takes practice and if it has not stuck for 40 years, it will be very hard to get proficient. My friend a cello player in Washington state, retired and after 6 - 8 months of not playing so much and thus reading, she told me, it took almost a month to get back. She was playing professionally and teaching for 42-45 years
Bach Cello suites - that will keep you busy. Ties specially over bars, get some Latin/Latin Jazz book
There is also theoretical or harmony analytical reading, where you probably work off chord charts. and in actual playing you sight read intended lines by the composer or arranger. As a bass player - even a jazz bass player, this is probably what you want as well. Unless you have to play the head - this is probably what you will need. How many songs have a bass line that is unique from bar 1 to bar 32 -
It's mostly /: :/ - Classic example is
"So What". This piece will challenge your counting chops. It's 16 bars of D Dorian, 8 bars of Eb Dorian + 8 back to D. If you can play the head for like 5 repetition and do not get lost on the count Then I would call it progress.
For counting and developing that internal clock. Set metronome to the 2 and 4.
Lastly get a teacher. I just got back to playing piano seriously and already have good reading skills. I play woodwinds as well and play arco on the double bass. I still got a teacher, someone younger than me. She corrected many bad habits and improve my reading of the piano which is polyphonic. A teacher is worth 100 youtube videos.
Good luck. In the words of Snoop Dog -"Read a chart everyday"
