I have similar gripes regarding the producer system, and was tempted at some point to just scrape it all altogether as I view it as more an aesthetic part of the war system rather than functional which makes it frustrating since it has potentially functional ramifications.
Something I personally wanted to see clarified is not that different tiered producers mean higher quality products but more along the lines of diverse products. The United States has the one of the most, if not the most, diverse military in the world. It has things from ballistic missile submarines, all the way over to nimble small fighters such as the F-16.
An idea I toyed with, with some remnants of it remaining buried in my notes on the producer system, is that assuming you have the arms manufacturing you can produce anything still. For example, while Inquista is not a producer, if it had the arms manufacturing level to be an Alpha producing nation it surely should be able to make its own 5th generation stealth fighters. However if it did not hold a producer slot, or purchase lets say the F-22 from the Duxburian Union it just can not make a F-22 copy.
One of the main reasons I support the producer system is for the most part people will just copy wholesale from real life. Let that be the MIG-29 or the F-15. Saying in the roleplay universe these RL-associated items such as companies or people (that is not the best example given the time we had David Camerons but w/e) belong to certain people we avoid OOC disputes about things.
I am all for nations coming up with their own equipment, however, I think that having base line producer nations is still a vital role to ensure that no long term disputes come up.
Taking your statement and ideas into account, what we could do is add in that classification tool to the spreadsheet as you suggested. If someone reaches lets say Bravo Tier, they are able to produce units and equipment equivalent to other producers in that tier even if all the RL-Counterparts are taken. They couldn't use the actual product itself, so for example if Inquista was bravo tier it could not make another Queen Elizabeth class aircraft carrier, but it would make a Juan Bernardo Fernández-Velásquez class aircraft carrier that has similar specs and /quality/.
That would help I think to one simplify the Custom Units/Classes/Whatever and the producer system altogether.
Appreciate your comments @Inquista just send me the sheet whenever you get a chance too o/
EDIT::
I should note that I want to encourage people not to rip off RL names of at least military equipment as much. For example, the Duxburian Union probably sells the F-16 in RP but it should not really be called the F-16 or anything. Just a bonus thought of mine.