How do I cut mitres/ bevels across thin edge of a joist?

Have a ground deck project design which is in form of an equilateral Triangle (all internal angles are 60deg). the main ('A') frame is going to be cross-braced to form the cross bars so it will look just like a giant letter 'A' with several bars across it. If it were rectangular I'd probably use joist hangers to tie these lateral elements (joists) into the side members of the main 'A' frame. But as the angle at which the lateral joists hit the side frames is 60 deg that's not an option. I need to trim the ends of the joists accurately across their depth for a good fit so that I am not having to make 'bespoke' noggins to fit between every joist: once I've set up my cross-cut saw to make 350mm long noggins I want to make the 'full set' on one set-up. So, how do I produce the required angle on the end of the lateral joists bearing in mind they are up to 3M long and weigh anything up to 10Kgs? The joists are mainly 100x50 C16 but I plan to fit a 150 deep joist at the front to close off the deck across its widest point for greater strength and allow a decorative fascia to be fitted. This joist will in the form an extension to the deck area and will follow the shape of the three sides of an octagon: the internal angles for this shape will be 145 deg and again will need to be cut accurately to ensure a good fit and appearance