Great work MasterFALE! I had started working on a similar table myself to see how maintenance can be influenced.
So in veteran, it turns out that at best by limiting yourself to one Tier IV (+1 or more Tier IIs), one could get 21% off whereas one Tier IV + 1 or more Tier IIIs gives a measly 10.5% discount.
If the discount percentages are tweaked a bit to actually give a more meaningful discount, then we'd get some noticeable control over the repair costs and I think the mechanic wouldn't feel as robbing. For example: one Tier IV has a base of 1550 credits right, then if each additional Tier III gives 5% and a Tier II gives 10%, by limiting one's options in a given game one would be getting 20% discount while still maintaining decent Tier III ship options, or a big 40% off for playing just one Tier IV and Tier IIs. A more noticeable control over the repair costs would diminish the negative aftertaste of being penalized to some extent while keeping the mechanic there to satisfy the publishers/devs.