Tech capital was changed in TA; it now functions as a planetary racial bonus.  What this means for you is that it adds on to your previous bonus, instead of multiplying it by 2.  For Tough, where there's no difficulty-based hidden bonus to research, this means, with the following:

25 (Altarians) +20 (customization) +20 (Technologists) +10 (Planetary Improvements) = +75%
+Tech Capital = +175, so 2.75x your base production.

In DA this would be +75=1.75, *2 = 3.5x your base production.
Essentially, you lose as much of your base production as your total racial bonus -is-, comparative to DA.
Granted, this fails to account for either the Research Coordination Center or the Omega Research Center, but their mechanics haven't changed since DA.  And, for the record, the Omega's description is misleading-it states it provides an empire-wide bonus, but only provides a planet-wide bonus, making it not actually worth the tech points to get.

You can alleviate this somewhat by playing on a higher difficulty, since difficulties higher than Tough get a hidden bonus (not shown anywhere but in your actual rp) to research, based on the research bonus you already have, which makes the breakpoint about 62% racial bonus on Suicidal (up until then, you're actually generating more research than DA).

But that's not the answer you're looking for.  Particularly as you would be spending even MORE money on research.

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Max pop is well and good, but it would help to know how many of them had actually maxed.  And how late in the game is this?  49% is a little low, I would tend to expect at least 59%, which is about 18% more taxes, for only an additional -22 morale.

Your planet count is a little low, but from personal experience I'd say you should have been okay, if perhaps a little tight-would you care to list your expenses?  Did you happen to do a large amount of lease buying?

Once your planets are maxed, you can afford to run at 21% approval, except the week of the elections (twice yearly), for which you'll want about 60% to guarantee winning.

Even up until that point, if you're not Breeder, 100% helps, but it's not that large of a difference from 41% if you need the money.  You still get normal growth at 41% approval (planet based, minimum, so your empire may deviate slightly on average).

More planets will definitely help (large map is big enough for now, just turn everything to abundant and watch what happens), but I still think there's something we're missing.

Perhaps it's the production planets-what are you building, and how fast?

There's nothing wrong with attempting to do everything at once, but I typically will only do research or only do building at one time-except if I focus on research with 100% military, which is something else entirely.

You don't necessarily need to stack up research on one planet to the maximum-some players have done that for kicks, myself included, but it's really not economically viable.  And since research is done on an empire wide basis*, it really only matters how much you have collectively, not per planet.  You do want your tech capital to be as high as it can be, yes, but there's a point where it's just not worth the money anymore.

I don't build anything on new colonies until pop reaches 5B. After that, I build two traditional factories regardless of what my focus for that planet will be. If I decide the planet is good for production, I build a starport. If it's an economic planet, I build some economic structures and a farm. My production planets stick with two factories and a starport at first while I churn out a few small-hulled constructors to make starbases and some defenders. Once I have some starbases and a strong enough military to deter would-be attackers, I research to advanced factories, I place my manufacturing capital, and I start building medium-hulled fighters. If I don't have any bonus tiles of any worth, I might build 3-4 advanced factories on the same planet, or I may develop nearby asteroids... Other than that, I don't develop much in the way of production. At this point, I'll typically research thru stockmarkets and intensive farming, and THEN I research through neutrality learning centers. 

     I'm thinking now that maybe I ran into some sort of bug in the first game I played because I haven't had nearly the same economic woes since then... 2 games now with the same settings and I don't even have to tech trade (okay, once early on) to stay alive.

Once you go over 20B things go down hill really fast, until around 25B or so. It is not linear, it is progressive.

If you could manage to make 25B people happy, you could keep 100B happy.

I'm going to try without reading the rest first of all 15 planets is not enough try a bigger map 48 or more is what you need i know its hard playing a lot of planets but welcome to rts try federalist instead of technologists if you don't have the finance its worse use the other for more constructive things like research later tell me your approval your approval needs to be between 70 and 75% lowering taxes for this approval is good it will benefit in the later game the torians are right in this trade with the yor they have the best research units with low diplomacy its cheaper anyways try researching discovery spheres build market places to fund research remember stock exanges you are only going to beat the torians in the later game can't beat the thalans on this try aggresively trading it will help you especially with the krynn,  arceans, and thalans again my opinion its great don't lower thalans torians or iconians difficulty bring the drengin arceans and dominion of korx to the iconian level add more races this is all i have on research the goal is 1000 bc per week you have to play a long time

One thing not mentioned in the posts above - best I could tell - was if you had any mining bases.  I have not played TA, but having a couple green economic bonus mining bases with lots of modules helps a lot in DA.