Costs of maintaining a large resin library

Who has experience maintaining a large library (5 or more I guess) of resins available? I know Formlabs printers are designed for easy interchangeability of materials. This a major plus for me.
But I am wondering how to make it work economically.

I guess the recommendation of one tank per resin is reasonable. Essentially it doubles the price of the first-ever liter of a given resin type, but ok.
But problem 1: what about colors? Each color needs a resin tank too?

Problem 2: The first time you use a tank a timer starts, and for some resins, after 75 days of exposure, you need to spend another 135 euros. This potentially means a worst-case annual cost of about 650 euros of “tray costs” for maintaining that particular resin available.

I thought, for less-used or aggressive resins, I could return the tray resin to a container, and rinse the tray to store it safely.
In fact, I bought 5 resins and 4 tanks, the idea being that Tough2000 and Elastic were going to be less used and I would share a tank between the two, which for the Formlabs salesguy taking the order, was a “perfectly good idea”.
The thing is, I’m not seeing ways to properly rinse the INSIDE of the tank to neutralise the attack of the monomer. Only for example mentions to “squeegee” it gently with the spatula before storying it in the case.
Does that really help to extend the life of the tray?
I found a couple of similar questions in the forum, but I haven’t seen satisfying answers.

I don’t think “Formlabs wants to sell trays” is a logical way to frame this. It makes much more sense to have a good value proposition and sell more printers and liters of resin!!
I think people using mostly 1 or 2 resins don’t suffer this a lot.

But the larger the library you want to have, the more cases of “rarely used resins” you’ll have with this risk of having impossible costs.
So what’s the good value proposition then?