Far from "one busy train a day", the Severn Beach line is well used, especially in the peaks but in my experience it's been well loaded all day, despite as has been mentioned the awkward frequency - numbers have gone up significantly in the past decade since the frequency improved and I'm sure would grow further if they moved to half-hourly, or more if some of the line is doubled. Less crowded after the Turbos replaced two car Sprinters but still busy. Might be a degree of undercounting of passenger numbers due to the lack of ticket machines and the guard/ticket inspector not always making it through.
Buses might be better/quicker for the centre on the whole given the location of Temple Meads but for connections at TM it's about twice as quick as the bus from places like Redland/Clifton, and that's before the unreliability of Bristol's traffic is taken into account. There's of course more and more happening around Temple Meads, and worth noting a lot of people get off at Stapleton Road/Lawrence Hill too, it's not all to/from TM. Quite a bit of school traffic too.
Buses might be better/quicker for the centre on the whole given the location of Temple Meads but for connections at TM it's about twice as quick as the bus from places like Redland/Clifton, and that's before the unreliability of Bristol's traffic is taken into account. There's of course more and more happening around Temple Meads, and worth noting a lot of people get off at Stapleton Road/Lawrence Hill too, it's not all to/from TM. Quite a bit of school traffic too.