• Power Stations (PS hereafter) generally perform to spec.
• most “problems” result from unrealistic expectations, which result from
  • failure to assess actual requirements before purchase
  • failure to consider how one will recharge before purchase
  • failure to read online specs/manuals before purchase

No. Read the power station's solar ("PV") input specs and buy panel[s] whose output specs are within the PS' stated limits.

Normal panels have MC4 connectors, so you will likely have to build or buy adapters to connect the panels to your particular PS.

you can run the DC output to a bus or fuse box to split the connection. The total current must be within the PS' DC ouput limit; see below.

There is no easy/cheap way around this. Buy a PS that has sufficient specs to meet your needs.

1. bad connection - 12v ciggy ports are substandard even on their best behavior.
2. the chassis voltage is varying, so the 12v volt outlet voltage is varying, so the charge rate is varying (I=V/R)
3. the power station is throttling charge rate

and if theres a way to have it always charge at 100w?

That depends on the cause, the power station, and one's ability/willingness to DIY a solution. And it can't be always, just more of the time.¹⁾

… then fix the connection. Clean/adjust/replace as needed. Anderson PowerPoles are a common upgrade.

If the wall charger output is 100w you could even run a cheap inverter from the ciggy port and plug the wall charger into that. It would be inefficient but we don't really care because the numbers are small and alternator output relative massive. This approach would work because the wall charger's output is usually at the higher end of the SG's DC input voltage limit.

If the wall charger >100w²⁾ you could get more consistent charging by boosting/regulating voltage up as described above. So if the wall charger runs at 15v one might boost the outlet's voltage to 15v.

If it's a larger PS that can accept 100s of watts it might be easier to run a semi-DIY relay/inverter setup.