"the reason most homebirths weren't successful for first time mothers was because MW were (rightly) very cautious if anything looked even slightly concerning and would transfer to hospital. "
So you've got two things there that I think you are confusing slightly.
Yes, the transfer rate is higher for first time mothers, partly because MWs are cautious and very understandably and appropriately, as you say, transfer early in home births to prevent issues at home.
Also, the women choosing home birth second and subsequent times have already gone through labour, so their pelvis has been "tested", they know the pain of labour and are less likely to be transferred for more pain relief etc, have had (normally) an uncomplicated delivery before so that bodes well for this time round, labour is normally shorter second time round etc.
But also, that study showed that first time mothers starting at home (so it included everyone who had a baby at home as well as those transferred) still have a higher risk of serious harm (9.3 events in 1000 births compared to 5.3 in 1000 births in the obstetric unit).
(Events were stillbirth after start of care in labour, early neonatal death, neonatal encephalopathy, meconium aspiration syndrome, brachial plexus injury, fractured humerus, or fractured clavicle.)
As for the fibroids, I've found this paper that might be helpful in stratifying the risk or for you as you ask questions:
"Pooled cumulative data suggest that postpartum hemorrhage is significantly more likely in women with fibroids compared with control subjects (2.5% vs 1.4%)" - but that does bunch together smaller (3-5cm) and larger (5cm+) fibroids.
www.ncbi.nlm.nih.gov/pmc/articles/PMC2876319/
Hope that is helpful.