I am a single person who doesn't drive and wishes generally to make at least one return journey when visiting a heritage railway.
Therefore, my plans always centre around how easy is it for me to get to the main departure / arrival point (which isn't always one end) of the railway. For the Llangollen, it would be Llangollen; for KESR, it would be Tenterden and so on. This sometimes (as now) means that some railways are inaccessible (i.e. the GSWR, because it's virtually impossible to get to Toddington by public transport), but this is, I know, a temporary thing and I'll be back there once the Racecourse station has reopened.
I don't generally take coach tours in the UK, but I do in France and Belgium when visiting the Battlefields because, although I could do some of these visits myself, the transport would be a problem in some areas. In addition, these are guided tours with very good guides, which is part of the trip and very much a bonus.
My other experience of coach tours comes from my work with Cadw, the Welsh Historic Monuments body. It was noticeable that the priority of many of the "senior" visitors on these tours was firstly a toilet and secondly a cafe; in fact, many of the visitors never actually visited the monument in question if we didn't have one or both of these! There wasn't much we could do about it, so we didn't try; it was something that we had to accept.
Another consideration was the amount of time that tours allocated to visiting a specific place. Generally, unless they were booked into an event at a monument, they didn't actually have very much time there, as they were travelling around a fairly large area each day.
These are just a few random thoughts, but I hope that you'll find them useful (and not too far off topic!).