Having read the question several times, it is still not clear to me if you are asking "when/where to search" (you have invested some time on "google, americanancesors.com, and Ellis Island") as opposed to "what research" you would conduct to learn/confirm the information about your ancestor's immigration (about which a ship's passenger list might be one of many, many sources).
You wrote, "The first part of my question is Q293 in the context of my ancestor (ie what else should I research prior to looking for ship)?
Finding aids (indices) emphasize names, dates and places, and little more. Nineteenth century passenger lists might not contain much more information. As with many/most historical materials, the finding aids and lists contain errors; rarely is the list information sufficient to directly prove the record is about your ancestor's immigration. Of course, all this assumes the list is extant and without omission (was complete as to each and every passenger without regard to race, color, creed, age, gender or social standing); it assumes a finding aid about the list has been developed.
For me, "conducting a search" is a subset of tasks that are part of the research in which I'm engaged. From the linked Wikipedia article, research "is used to establish or confirm facts, reaffirm the results of previous work, solve new or existing problems, support theorems, or develop new theories."
If you are simply trying to exhaust available finding aids based on what you believe you know, below are some suggestions:
- Use a research log or journal to record each search, your search criteria, and search results. As part of your log entry, I encourage you to study and note the information about the database or collection in which you conduct each search and the date on which that search was made. In my case, "search results" includes making notes about specific entries in the finding aid that look interesting.
- You can probably improve upon the "name(s)" (and thus the variants) you are applying in the search. (a) You wrote, "Franz Joseph Kerber was born in Jan 1825 in Munich, Barvaria [and he immigrated to the US]." It is not obvious from the question body or linked WeRelate file how you know this information, thus "how reliable" that information is. (b) From limited research, I didn't find the surname "Kerber" to be prevalent in Germany. It seemed to me that Koerber might be a variant, but it too was not prevalent in Germany. The name Körber, however, did seem to have some prevalence in Germany. You may want to correspond with contributors to the site, Kerber Surname Meaning and History; see "Kerber is variant of Körber," and "Körber and Jewish (Ashkenazic): occupational name for a basketmaker ... from an agent derivative of Korb ..." I'm sure there are other materials on this topic. (P.S. I did tinker about this man and did not find references to his middle name or a reference to "Franz." I found Francis, Frank; often a middle initial, "J." Do recall one entry indexed as "Frankz." The source of your information about his name wasn't obvious to me.)
- Your date search parameters (1826-1858) are probably not correct. Your WeRelate file reports he married at Greenfield, Mass. and that his first child was born either there or at Scituate, Mass., on 5 May 1852. (Again, we don't know how you know this.) These date parameters are particularly relevant to the part of Fortiter's Q293 answer where he writes, "What is the earliest record you can locate in his or her new home?"
A little separately, I read the Person Talk page about this man at WeRelate, in which you report about negative searches. Yesterday I searched some of the same big name databases that are listed there, and other alphabet soup sites; I came away with a long list of records and information that would be relevant to this man. In the process, it seemed the basic framework exists from which you could develop a good research plan about him and his family. Conversations with the genealogical societies, libraries and town clerks offices at Greenfield and Worcester could add even more value to such a plan.
I'm not saying this is the case, but when I looked at the Talk Page comments, it seemed you might be searching narrowly ("Frank/Franz/F Kerber") in broadly compiled (humongous) electronic databases. My approach is different. I prefer to seek a record where I expect to find it. I like to work from what can be very specific "known" (confirmed) information, but if I don't find the record where it should be, then I search broadly (dates, names, places, even "browse the pages") but still in specific resources. The better defined the resource, the more attractive/valuable it is to me.
You wrote, "The second [part of my question] is what resources beyond those mentioned would assist me in my search."
- Some of the most valuable resources you can work with are research materials you create yourself--research logs and/or journals; research plans, updated as necessary; documented interviews with genealogical societies, libraries and town clerks.
- Along the lines of no. 1, consider a makeover of your WeRelate file. (a) Try to keep a careful record of the many different events about which you learn and cite all your sources (even if you don't apply a formal citation style). (b) Although the WeRelate file may be misleading, it's possible you are mostly engaged in researching your direct line rather than all the individuals in each ancestral family. You'll learn much more of the whole story if you research about the entire family group.
- Learn "about" the events that are the focus of a search--German immigration and emigration in the period XXXX-XXXX (how, what, when and why); German communities at Greenfield and Worcester, and settlements of those communities at the relevant time period; naturalization in the United States and specifically in Massachusetts in the period XXXX-XXXX.
In a comment above you wrote, "I only have my late father's notes ... He didn't keep much detail (if any) on his sources. When he was young, he also corresponded with his grandfather on genealogy so some of the data comes from those letters which he kept."
This again may be a difference in our approach to the work. I think it is a good idea to first research to confirm the information contained in inherited materials, carefully documenting what I find and exhausting related resources in the process. Before long, I am on my way to developing a body of evidence, for which the family history/tradition is only a part. Then I can work from the more complete research materials to extend what "I know" (rather than what someone else thought they knew). I expect my research about the known to lead me to information about the "unknown."
P.S. You wrote, "... particularly rich" in resources. Massachusetts is a wonderful place to research. For a family of five children, there could be give or take fifteen vital records just about the kids lives; more when you add their children. One would expect at least three more vital records for the immigrating parents. If their parents and siblings immigrated, you might double the fun. Add news items and obituaries, census records, deeds, probate, court, cemetery, naturalization, etc. When you add the in-laws and other friends, associates and neighbors, then wow!