You might think about doing both countries? There’s frequent ferry service between Dublin and Holyhead in Wales, or between Belfast and a couple of Scotland locations. We did the Dublin - Holyhead crossing on a huge cruise-ship style ferry, the James Joyce, with a tour group that from Holyhead took us to Carnarvon in Wales, and from there to Liverpool, Oxford and London. There must be transportation options from Holyhead to Glasgow.
Ferry routes to, from and within Ireland - the crossings captured Ferry routes to, from and within Ireland - the crossings captured
And it’s not true that being with a tour keeps you confined to only the people on the tour group. I’ve been on 3 tours over the past 30 years and in all of them there was at least 1 independent meal a day, with free afternoons and/or evenings to explore on one’s own. If you really look for one, you might find an all-inclusive tour where every meal is covered, but I haven’t seen one.