Starfinder is explicitly far more focused on balance from the ground up than pathfinder ever was. While certain gaps are inevitable, it's going to be much closer than other systems, and not what I'd call "tiered". They crunched the numbers a lot more in the development of Starfinder.
That being said, if you're talking combat damage, Starfinder did not change the Pathfinder truism of two-handed melee being king. To steal a graph from elsewhere on the site:
That is just raw weapon damage, not factoring in any special effects, but it does illustrate the point.
The issue with "linear fighters, quadratic wizards" found in 3.x and Pathfinder has been at least mitigated on a couple levels, reducing the tier-boosting that full casters used to get. A lot of utility that used to belong to full casters now belongs to technological items, and low-level spells don't scale up in power with level. So if you want to be dealing higher level damage, you need to be burning higher level spell slots.
In short, no, tiering isn't gone, but they're a bit narrower. Some classes take a bit to catch up (like Operative lagging somewhat before level 7), but while there may be some poor builds, there's not really any bad classes anymore.
No, there isn't
As for this date, there are no published rules, or 3rd party that I have heard of, that introduces any rules for building bases. However, I had this discussion with a community member of the Starfinder RPG discord server, where the group was a rebel group using stealth tactics to undermine a stronger enemy with starfighters and a base of operations to return and rest, which is a cool idea for a star wars rebellion-type game.
The idea we came up with was to split the party's BP with a few smaller ships, which would require the GM to also balance combats accordingly or they would get stomped by enemies using bigger ships, and use part of that BP to build the non-combat ship accessories for their base of operations, like med bay, engineering, and science lab.
Of course, since they won't have ships, the BP could be spent entirely on their base/colony.
Downtime rules
I also suggested that he would look at the downtime rules for Pathfinder to gather ideas from, but considering the theme and a complete difference in currency, we can't just use that system without heavy modifications. The main differences here would be on the currency and flavor of buildings and teams.
A proper scale, at least for the early levels would be to consider every gp as 100 credits. So, a day working to get labor or goods would cost about 100 credits, and produce enough capital that is worth about 200 credits.
This would allow them to build several structures, hire teams to do particular jobs and they would be able to handle the currency generated so they could invest on new structures or building new gear, probably building the necessary so they can fly out of the planet.
Kingdom Building rules
A lot of things in the Kingdom Building system have to be discarded, and the remaining will have to be modified. In concept it may sound easy, the general idea is easy, but the execution would take you a lot of time and they will be playing a different game, much better suited in the rules for a Pathfinder game using the Technology Guide and Starfinder as inspiration and conversion ideas.
The kingdom roles will have to be changed, as they are written with a monarchy in mind, which isn't well suited for a sci-fi setting, as even modern monarchies are different. Hexes become irrelevant with highspeed travels, as bikes and cars are nearly as fast as airships. Loyalty become irrelevant, people really have no loyalty to their leaders, but that stat could be renamed to Satisfaction or Happiness without a lot of issues. With no hexes, you need a new form of controlling the DC. The DCs and skill scaling are different in Starfinder and you will have to create a scaling DC table similar to the one found in Pathfinder 2e. "Science" or "Technology", to me, are just as important as Economy in a sci-fi setting, so you would have to come up with a new kingdom stat, which would require adjustments on the kingdom stats too.
The kingdom building rules are intended to work with multiple hexes and settlements, and while they work for a single settlement (a colony), they will be limiting themselves to a fraction of that system's potential.
I didn't even get into the buildings and I see myself writing a civilization/endless space adaptation to Starfinder before this is done. But don't take this as un-motivation, because if that was what my players wanted, that would be exactly what I would do, even if it took me a few days of tinkering and tweaking of those rules.
Best Answer
There are detailed rules for parties with multiple ships now that the Starfinder Starship Operations Manual is available.
The previous answers to this question are well done, and they were accurate; however, they have been overcome by events with the Fall 2020 publication of the Starfinder Starship Operations Manual. This rules expansion contains a section on what is called "Squadron Play," in which the party is allowed to have as many as one starship per character. It contains detailed rules on how to balance the tiers of multiple ships, upgrade them whenever APL increases, and how to create "HQ ships"--large ships in which smaller ships can dock so that the players can interact face-to-face, travel long distances together, spend down time, etc.
In addition to rules on building and balancing multiple ships for a single party, the section also details new crew actions designed for cooperation between multiple ships in combat, such as flanking and flying cover for a wing man. There are also new starship upgrade systems designed for multiship parties, including a system that allows multiple ships to fuse together into a single large ship and break apart again into the constituent ships.
The book also has a section on armada combat, which is basically mass starship combat in which each player can control entire units of ships. Armada combat is for specific engagements in which the player characters find themselves taking military leadership roles, not for long term campaign play--which I believe is the kind of multi-ship party the original question was asking about.