Getting to Sofia!

I traveled overland from Sofia to Montenegro on a bus by way of Nis, Serbia, but I doubt that anyone could call that connection "easy". I think you'll find that there are no expeditious ground connections and the only easy way in or out is by air. Take a look at the Wikipedia web page for the Sofia airport. It shows which airlines have direct flights between Sofia and other cities (though not necessarily daily).

There is a great train connection from Belgrade. And if you are in Montenegro the train ride from Bar/Podgorica to Belgrade is supposed to be stunning. Although I have booked the overnight on that route. Then from Belgrade to Sofia by train again.

I view these obtuse connections as opportunities to see another place even if it is for a short time (overnight).

But other than that the hassle, cramped, rude and time consuming city-airport-flight-airport-city dance will get you there faster.

I had a great Air France flight from Paris to Sofia last year. You will love Bulgaria. Stephan is an amazing tour guide.

I flew British Airways from London to Sofia.

In general, from almost anywhere, a flight will be simpler than ground transport; see www.skyscanner.com.

If starting with the train from Belgrade, service is not currently running between Nis and Dimitrovgrad, so you'd probably overnight in Nis (train arrives there at 23:00) and connect to a bus the next morning (or you might take bus the whole way). And while the direct train service from Thessaloniki to Sofia did reopen last year, schedules are not currently available (but there are several bus companies operating). From Istanbul to Sofia, the overnight (only) train requires getting up for a border check about 2:00 a.m.

I spent 3 nights in Skopje, Macedonia and took the bus to Sofia. Bus was comfy and it took about 4-5 hours (we had over an hour at the border, which was a tad scary as the only American on the bus and female). Not only is Macedonia worthwhile (I need to return to visit 2 other cities), but telling people that you are going to Macedonia AND Bulgaria is awesome!

Thanks for posting this, George.
I'll be in Bulgaria, briefly, next month as part of a river cruise, but the RS " Best of Bulgaria" tour is at the top of my list for future consideration. I had the pleasure of meeting Stephan at the RS tour guide event in Edmonds in late January. A charming fellow.
I'm hoping to combine " Best of Bulgaria" with another RS tour (either pre or post) to eliminate all this transatlantic and transcontinental hopping. Maybe something Italian or Greek.