Does each tier of fishing rod have it's own base durability, presumably that increases as the tier # goes up?

If so, do we have any idea what these numbers are?

so yes, a pine rod is certainly stronger than a splintered rod. No we dont have any numbers

Interestingly, the higher the total durability of the rod, the slower you will catch fish. So if your goal is to catch lots of fish, using lower tier and lower durability can often be a good thing.
In general you want to hit the breakpoints as soon as you can. Catching level 15 fish in a well is worth waiting for a good 60%+ fishing event at rank 9 fishing and using a 90+ dura level 15 or 20 rod for.
Catching 1x level 15 fish in a well is worth more than 2 level 10 fish outside the well imo

Two more questions:

1) Is a +20 durability fishing rod equal to a baseline fishing rod one tier up?

2) is there are larger, consolidated fishing guide I can reference?

1) If we knew that I would have answered that the first time you asked. However i don't think this is how it scales, i'd consder the base rods to be a multiplier. This is more noticable on later rods, but a level 20 rod with 50 dura is much closer to a 15 rod with 125 dura than a level 15 of 70 dura.
2) https://forums.d2jsp.org/topic.php?t=80825020&f=74&o=0 this is the one i learnt from and asked questions about.

Thank you, reading that guide now.