I've made a quick & dirty tree incorporating all the information provided above using GenoPro.
Color codes refer to cM matches. Relationships & percentages underneath the boxes refer to the Unknown Great Grandparent being a son of Ellen Nielsen & Niels Dahl; parenthesized relationships & percentages within the boxes refer to OP's hypothesis of the UGG being a son of Ane Nielsen & Rasmus Johansen. The percentages are based on results from dna-sci dot com, as they also use segments in calculations.
Mind that other relationships that fall outside of these particular percentages are still possible, though less likely.
Thank you so much - that is clearer to me.
The son that was thought to be the likeliest candidate was Martin DAHL but I see no evidence of any of them being in the area required. I also still firmly believe that my GF's father was a "seaman" but these were all farmers as far as I can tell. I don't really know what Martin did in his younger life though and there was a M[artin?] Dahl on shipping record but it has born in Norway for that seaman. I must check that out again. The first candidate suggested was younger brother Christian but he died in 1889 and I think that was only because of his age. I also considered the father Niels but he died 1900. The cousins are even younger so maybe not likely.
The highest match was Birthe 1940-2022 and my diagram was initially based on the relationship between Allan and Birthe's father Svend Rasmussen as Allan had him in his tree and MyHeritage drew up the connections.
I am even considering the fact that Allan & Dan's Ancestor, GGM, sister to Svend's father, Niels Clausen Rasmussen, also married a Nielsen so possibly VERY close connections within the family(s) which would inflate some connections - so maybe the MyHeritage connections are not higher but inflated compared to the Ancestry ones. Also Ancestry trims back matches cMs with its Timber algorithm so maybe
Also back to Martin DAHL - I have a DNA match on Ancestry to a Great grandchild of his through his daughter but it's not particularly high for a possible 2nd cousin at 16cM across 2 segments but trimmed back by Ancestry from unweighted shared of 25cM with longest being 16cM. I have equal or higher matches to Martin's siblings Great grandchildren. They go from 8cM [Dan's sibling] to 43cM or so descendant of older brother [Anders] Angus or maybe Neils Peter DAHL, can't remember. Is it Ok to invite you to View or collaborate on my results? I have Access to others for viewing only - one in this line.
UPDATE: Here is the record on Ancestry with a Martin Dahl aged 21 -a Bookbinder from Norway and also an Alfred Dahl aged 30 a Coke Dealer? of NS [Nova Scotia?] on board 'Alaska' June 30 1890 - Passengers?
Funny thing is my father was a Coal & Coke expert and worked at BHP Newcastle Steelworks, becoming Superintendent of Works Research until this department closed & he took retirement. Not sure if this man from NS was that kind of a Coke dealer or ?? Just interesting and his surname was Alfred Dahl. My GF's middle name being Alfred. Although the thought was that my GF was named for his uncle - John Samuel Gregory had a brother James Alfred Gregory 1859-1939 died in Channel Islands. My GF was the only child named directly after a relative that I can see in the Gregory line. My be deliberate as he may have known not his father ?? The name follows through my family another 2 generations.
Here is the link on Ancestry:
https://www.ancestry.co.uk/discoveryui-content/view/7547399:7488?tid=39001678&pid=162486859838&hid=1031965501248Thank again for your time and sorry I took so long to get back to this. I really appreciate it.