Your straight vertical drop into a clay pipe is iffy- as others, you'll get solids splattering, separating from the water and potentially building up a restriction or blockage. Situation will be improved if the shower, basin etc use the same vertical drop but it is still iffy.
Usual deal when entering an existing manhole is your new pipe must be at invert level (ie bottom of new pipe level with bottom channel in manhole). On new placcie manholes the side entries tend to be 50mm above invert, if you had one of them then obvs you go for the side entries.
So with your sketch you'd be sensible having your drop from the bog hitting a rest bend then onwards to your manhole. You've still got an unroddable vertical from loo to rest bend...a manhole under the patio is tolerable as long as the slabs are loose laid (do yourself/future owners a favour, write the measurements for manhole position down, laminate them & hang them from the stop tap), rodding point under a laminate floor (which would take a certain amount of effort and potentially damage) isn't really on.
No idea about the durgo, don't like them, never used them