A number of questions raised there

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Pro used to be a separate package. Now it is a £5 add-on to Value or Extra, and removes
the traffic shaping from them, so everything should be line speed as per the old stand-alone Pro product.
Plusnet Fibre is good, like most BT-based FTTC products, but has an 18-month contract and only (at the moment) the 2Mbps upload option. Allowances are 40GB and 120GB. I don't know if the Pro add-on is applicable to their Fibre, as I read the page I link to it probably isn't.
Have you checked FTTC is available on your line?
Plusnet line rental is just the rental is paid to them. The line stays as a "BT" line for all migrations. In the past, you couldn't
order Plusnet line rental without having their broadband, but they let you keep it if you then migrated the broadband away. (Except if you moved somewhere that demanded the line as well, such as BT). I think this is still the case, but I'm not sure.
You may be misreading their website. The line rental is only payable to them if you take a broadband and phone package. If, for example, your line is with BT like mine, and you wish to leave it there, just take a broadband-only product

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My broadband basic info/help site - www.robertos.me.uk
My domains,website and mail hosting - Tsohost.
Internet connection - IDNet Home Starter Fibre.
Live BQM.
"Where talent is a dwarf, self-esteem is a giant." - Jean-Antoine Petit-Senn.