Most likely you overestimated your hit chances against a fortified enemy on home terrain.
The combat mechanics broadly include:
- Base 20% chance to hit, modified by the usual commander and etc. capabilities.
- Fortification, which can be as high as 6 for infantry if they have engineers/CON somewhere in the rear areas, and this divides the hit chance. Without CON max would be 3.
- Terrain: Savannah offers a 1.15x fortification multiplier and a 0.75 to-hit multiplier. Combined these would be about a 0.65 modifier
- Environmental factors, which don't seem to affect you here as you've accounted for them
So depending on the presence of CON in the rear echelon you probably have either 4.34% or 2.17% chances to hit depending on whether the units are self or max fortified. This would be somewhat higher against armor which has worse fortification ability. Your combat report indicates that your infantry scored 94/2160 = 4.35% which is just about bang-on. Your heavy tanks scored nearly 10% but with so few shots the variance would be quite high.
Glancing at your units I notice that you seem to have weapon tech of 10 but armor tech of only 6, which means the enemy units are penetrating quite a bit more reliably than you'd like. You're right that the enemy is behind you in tech but with base attack 8 they can penetrate and kill you fairly easily which you're seeing here. On a related note, weapons like HAV are often overkill especially when you prioritize weapons over armor tech, using MAV and other smaller weapons will allow you more firepower per tonnage unit which can help.
Additionally you should consider deploying more infantry with some form of PW-type weapons. They may not be as tonnage-efficient for killing things, but you can fit more actual elements into a formation which means the enemy must take many more shots to kill you, this will help overall survivability.
As a last note, you say the enemy troops are 180,000 tons. How did you verify this? If this is based on what you detected from orbit, bear in mind that orbital signatures for ground forces are reduced by the fortification level. Here if the enemy is at fortification level 3 you are actually seeing 540,000 tons of enemy troops for instance. In this case you are not only overmatched in terms of armor vs enemy weapons but also outnumbered, and it is no surprise that you are taking such losses!
In general to plan a ground assault, you need to start with the enemy ground force signature from orbit, multiply it by an assumed fortification level (either 3 or 6, usually, depending on if you want to assume CON are present or gamble that they are not), then figure out the various terrain and environment factors and multiply by that as well. That will give you approximately the
minumum force you need to land to have an even chance, and you probably want 2x or 3x that number to ensure a dominant victory. One might note that this makes homeworld and other major fortified bodies very difficult to assault, and the answer to this of course is orbital bombardment which of course comes with a high price in environmental damage.