Would it make sense to add some inertia to the hydrosphere type or have larger hydrospheres provide some inertia to planet temperatures?
It's probably not very realistic to have planetary sized bodies of water swap from frozen solid to steam and then back again in a matter of days.
Interesting idea. In the past, the change from liquid to vapour was instant because it was always the result of terraforming and therefore likely to be permanent. Maybe the change should be gradual in the same way as condensing (0.1 atm per year). That would solve the problem of drastic changes in short orbits while leaving the option of different conditions in long orbits.
Liquid to ice and back is less of an issue because it only affects albedo and doesn't introduce a sudden 2.0 colony cost (and I already include albedo changes in the min/max temperature and min/max colony costs). I could perhaps track the percentage of water that is frozen, rather than having separate states, but in reality the water to ice process happens quite quickly so probably not worth it.
Temperature changes could be gradual as well. Planets heat up because radiation from their star hits them, and heat is simultaneously escaping the planet by radiating away. If more heat is radiating away than is radiating in, then the planet is cooling, otherwise it is heating up. One interesting quirk is that the amount of radiation emitted is proportional to the fourth power of the temperature, so hot objects radiate away heat faster than cold objects, all else being equal. (On the other hand, emission doesn’t really become an efficient process until you hot enough to glow in visible light).
With some interia in the temperature, the planet in your example would probably have a narrower range of temperatures. Especially since it spends a lot less time near the periapse, and more near the apoapse.
Perhaps For extra fun, you could actually implement the single–layer model, where the ground absorbs part of the incoming radiation and reflects the rest (due to albedo), the ground emits radiation, part of which is absorbed by the atmosphere. The atmosphere also emits radiation, some of which goes into space and some of which is reabsorbed by the ground. Normally you solve this for an equilibrium condition, but with an elliptical orbit the numbers wouldn’t always balance. You would need to track the atmospheric temperature separately from the ground/hydrosphere temperature, which could be a lot of fun. Plus you would get to decide what temperature new gasses from the aether are brought in at when terraforming!
Another thing you could do is to take into account the latent heat of the hydrosphere as it freezes and melts. As you know, water that is freezing stays at 0℃ the whole time. The heat that is in the water is emitted to the surroundings, but instead of dropping the temperature of the water, this causes a phase change instead. Likewise, when melting the ice absorbs heat but the temperature does not go up. If that planet has any significant amount of water, it would buffer the temperature quite a lot: as the planet heats out towards the apoapsis, the temperature of the ground, the hydrosphere, and the atmosphere would begin to drop. The hydrosphere would begin to freeze, dumping heat into the atmosphere and helping it to stay near 0℃. Thus the colony cost would not go up right away.
