Norski Epoxy Timber Sealer 500ml - Bunnings New Zealand

Part B (?or is it A?) is maple syrup coloured so when mixed with the other part it ends up amber coloured

I wouldn't use floor sawdust, too hard to know what's in it.  You could sand the slab on a big clean tarp and use that to collect the saw dust.  Most people go for a peanut butter consistency for filling compound, which is able to be sanded to a paintable smoothness.  You might want to experiment with a small quantity to check it can be sanded to texture suitable for clear coat if that's what you want to do.  If it's too coarse, then use less sawdust.

I think epoxy sealing a macrocarpa slab will be prohibitively expensive.  If you don't seal the whole thing, then I would use epoxy as a primer inside ana larger holes you want to fill. Just two coats.  Again: Hot-coat, including the filler.

If you've not worked with epoxy before I would recommend: -

• Acetone for clean up
• Disposable cups - I buy party cups in pint and shot size
• Disposable stirrers - I buy ice block sticks or use small bits of scrap timber.
• Disposable brushes or rollers and trays
• Use a portable digital scale inside a zip-lock bag for weighing resin.
• Measure each part into a separate cup and don't fill more than 40%
• When mixing the resin parts, stir well then pour the combined mix back and forth between the two cups a few times stirring each time.  When stirring, scrape the sides and bottom of the cup.
• In between coats, put resin inside a sealed box and put it in the fridge to slow down cure.
• If you're working with pigmented liquid epoxy (no sawdust) then I've noticed that people use heat-guns on larger areas, just after pouring to help get a smooth surface.  I've not tried this myself.  I'm sure there are plenty of how-to-safely vids on YT.