Think the issue is that this is a one-size-fits-all system that tries to abstract similarity of "levels" between countries that historically were nowhere near on parity with what the game would consider "research".
For the most part in WW2, research outside of HoI4's majors tended to be more scientific and civilian-oriented in nature (Electronics and Industrial techs, basically). As far as the war side of the equation, for the most part, I would say most countries did not "research" more than a couple of technologies historically, and many did literally none at all. Of the countries that did "research", it's pretty disproportionately wrapped up in Infantry Equipment and Artillery.
If you look at the countries that "historically researched" naval techs during this time, for example, you pretty much just have the US, UK, Germany, Italy, Japan, and to a limited degree, France, the Soviet Union and Sweden. Look at Air and once again it's just Majors-sans-China except this time the random minor that "researched" a tech was Romania (and Argentina, I guess, if you count immediate post-war designs). Armor, very similar story. I could go on.
Things get even worse when you look at how even for the majors that changed during wartime. The UK, for example, almost entirely "stopped researching" in in-game terms between early 1940 and the start of 1944. You had a handful of armor designs that went into production after the fall of France, you had the PIAT, and you had the Minotaur and Colossus classes (new Destroyers, though I would argue based on existing technologies) More planes, at the very least, but the majority of them were 1939 designs that just took longer to get into service (even the Spitfire is secretly from '39!)
That said, I would say, for the most part, that the only countries researching more than a handful of techs during the main part of the war are the US, Germany, the Soviet Union, and, to a much lesser extent, the UK, Italy, and Japan.
The real key issue is that there is a missing factor here. Beyond their continued production of pre-war "tech", the bulk of "new" during the war for the UK comprised of Lend-Leased and Licensed technology, and even after many passes, that's still not something that really works all that well in HoI terms. That's a rather happy example for HoI as well, considering that the UK at least still made some new during the time, unlike, say, Yugoslavia, Romania, Bulgaria, the Commonwealth countries, China, Spain, etc., which, outside of a few specific "techs", essentially went the entire war with nothing but their "starting equipment" and lend-leased/licensed stuff.
Part of that is from the fact that this is a video game that needs to be fun. If the UK explicitly had to rely on Lend Lease and Licenses to the degree that it did in real life to avoid having to just use 1930s technology through the war, the only real way to do that would be to block you from conducting research or make it so slow as to just be a trickle. Nations like Yugoslavia would just be depending essentially entirely on other people both making good equipment and letting them have it. Understandably, that's not awesome game design.
But because of that, everyone kind of has research and design as if they were one of the major nations in the world- and to double the need for that, it can be the case that Hungary becomes Austria-Hungary or Xibei San Ma becomes China all of a sudden. So that said, I don't think the system could ever be reasonably completely upended, without ruining something on one side of the equation or another. Industry Becomes Tech is also awkward, because it's not necessarily true in practice (again, the above British example would kind of ruin that), and also feels more out of sorts without the glut of Civilian-based techs 3 and 2 had.
Think the real deal is, if Licenses, Lend-Lease, and Equipment Buying finally get smoothed out for real, tech slots become a massively smaller issue. The real problems as I see it are, therefore...
1. Licenses are completely non-worthwhile to use due to their heavy penalty on production. Changing that mechanic full stop would break multiplayer games, so something needs to be done where licensed production is very worthwhile for minors while being a smaller luxury for the majors.
2. Lend-Leased equipment is hard to use because if the stream gets cut, it's not easy to just immediately stop using it and your units will suffer. If you're getting lend-leased Tanks as a country that isn't making them (or a certain kind of them) and the flow gets cut, you can't just wear out your existing stockpile in any way.
3. Equipment buying is probably reasonably balanced for the buyer, especially the sort of country that would be most often buying equipment in the game, but the seller still doesn't get enough for parting with what is a key component of their war machine, and thus most of the world market consists of massively obsolete junk and equipment in too small amounts to actually do anything with (it was very common to order 18 tanks or some number like that in real life, can't exactly do that in HoI4...)
Solve these issues in a way that doesn't just directly implant steroids into the MP Allies and having up-to-date research on everything becomes far less important, and the research slot "controversies" probably drop considerably. In particular, recall that, as obscure as the mechanic is, you do get research bonuses for using licensed equipment (even if it could definitely afford to be a bit moreso for more obsolete techs), which fixes the oft-mentioned "inventing biplanes in 1950" issue.
...Though, that being said, it definitely does feel like research in general is on the slower side, and I think it would be good to take a look at either lowering the research times for some hard to justify techs, or at least handing out some more Support Company MIOs.
