Z17925 is Germano-Celtic
Y17954 and downstream clades are an evolving story.
I don't know about other testing services. Check at Family Tree DNA, as they could well have more helpful SNP testing for that branch of I1 by now.
Haplogroup I1 was born on or near the North Polish Plain, and then most of it went to Scandinavia and had a big founder effect. 99% of I1 men have DF29 and the others have Z17954/ A8100. It has two main branches, Z131, which also went to Scandinavia, and Z17925, which headed south into Germany, crossed to the Rhine delta region, and then migrated along the Rhine and its main tributaries, along the coast of France, and across to eastern England and Scotland, and the coast of Wales. It clearly followed the main trade routes during the Bronze Age, and the attraction in western England was tin. They may have continued moving in Celtic times. Most places where they lived southern Germany, France, England and Scotland were Celtic at some point, those along the Rhine lived in the Celtic center when it was Celtic. One group seems to have moved down the Rhone on a major trade route used by the people of central Europe and the Rhineland from Bell Beaker through Celtic times. Same people, evolved through a stage of cultures.
Most people in this clade belong to its major sub branches. Knowing the sub branch might pin down its ethnicity more closely, might not.
Unfortunately even Eupedia's own Haplogroup I1 page is sparse on the details for I-Z17925 and the downstream clades.
View attachment 10131
It simply labels I-Z17925 as "North Alpine". There doesn't seem to be a lot of information (so far) on the non-DF29 members of I1.