I have already said that what little potential these trees have for bonsai is way in the future. I would pass these ones and keep looking.
My experiences of flat cutting the bases of olives has been positive. Occasionally one will die but the vast majority survive.
olives can exist with remarkably few roots for a long time. It often takes 2-3 years for new roots to fill a pot which is far slower than most other species I've worked with.
I agree that the top is also a huge issue. Probably far more of an issue than the roots. Reducing usually means reducing height but we have also been talking about using dead wood to reduce visual width and improve perceived taper so some width reduction is possible in some cases. Your other smaller trees have all had the trunks cut to reduce the height so you know what the results look like. Just multiply the dead stumps you have by 10 to visualise how these might look after trunk reduction. Although olives do bud reliably after trunk chop they are also relatively slow to grow so it can take many years to grow a good top on a stump. The larger the stump, the larger the top needs to be and the longer it will take generally to develop a new canopy. ramification on larger trees also needs to be better to fill out a larger canopy so that takes even longer.
The good thing about larger trees is that it can be easier to hide some faults and problems with form and shape don't always stand out so much.
Larger trees can be challenging. When repotting time comes, can you lift the tree out of the pot to prune roots and replace the potting mix? Can you rotate the pot each week to get sunlight on the back to maintain healthy branches all round? Do you intend to ever show the tree? What sized truck will you need to transport it to show or sale? Growers of large bonsai also cultivate friends and acquaintances with strong arms and backs. My days of even considering large trees are well gone.
Large bonsai pots can be difficult to find and are exponentially more expensive that smaller pots as they seem to be much more difficult to make and fire but don't worry about that yet. You won't need a bonsai pot for either of these for another 10 years or more.
Large trees also take far more time to maintain. There will be far more growing tips on most large trees so trimming can take hours each time. Wiring can take days.
Larger bonsai do look impressive. I enjoy looking at them but you do need to know all the drawbacks before committing to trees like these ones you have found.
Thanks for taking the time and all the explanation, as nice as those trunks are indded makes sense to skip those.
just to answer your questions and not arguing the purchase
When repotting time comes, can you lift the tree out of the pot to prune roots and replace the potting mix?
No both trees are big and I assume to heavy. Plan was to have it flat cut and repoted in the nursery so I can use their saws and get the help of their workers even.
Can you rotate the pot each week to get sunlight on the back to maintain healthy branches all round?
confess this is not something I knew I should do. some of my trees get sun all over but others don't. will start doing this to my other trees. rotating the pot will require help at that level and I doubt when a big tree buds heavily i can get away without shading even if in a well exposed spot.
Do you intend to ever show the tree?
No bonsai community or shows here unfortunately (not that any of my trees is show quality as well to be honest. all my trees are for my own enjoyment and learning and for displaying in my garden.
still if I get a large tree worth the effort I don't mind getting it and relying on Nursury staff support (very close to my place and they sometimes do some maintenance on my small garden anyway)
Again thanks for all the time and excellent explanation. will keep looking and hopefully will find a nicer tree to start with maybe aronud half the size of these and with some roots.