Part time is a thorny issue, even in countries that have the absolute right for every employee to demand to be put on part time (but not for the company to force them).
First of all there is always a financial side that the company will use at some point or another during negotiations: While your working time decreases by a fixed amount, your productive working time decreases actually more than that percentage as the required amount of training, and days put aside for that, remains the same, no matter how little you fly.
And then there is of course the issue that there are as many part time schemes as anybody can envision. I have worked on monthly based part time (1 month off, 1 month work for 50%, different ratios for other percentages, including special winter deals like take one month part time, you get a second one fully paid), a certain number of days as part time per month, reduced hours per year, special roster patterns and so on. And in a previous company basically every employee could write their own part time scheme and it got approved, if not from the company directly then later on in front of a labour law court (yes, suing your own company was commonplace and no reason for dismissal or career disadvantage).
And as others have mentioned, especially if you have a mix between full time workers and part time workers, there is a tendency of companies, if the contract is not written carefully, that the part time workers have to fly more during their working days.
And in the end of course it depends all on the local labour laws, and those differ wildly.
If the company wants to introduce part time because they have a surplus of pilots but do not want to fire them, there is already a common goal, which can help negotiations. Usually, in that case, a flat reduction of working days is something both sides can live with, so every pilot works less in terms of days. If you are paid by the hour (not all pilot contracts work that way), then of course you have to balance hours across the pilot pool, and that can be difficult, especially if you have quite a variance in sector lengths. In that case i have worked in the past with a 3 month gliding period in which the hours have to be balanced across all pilots, but even that has some issues depending how overtime is calculated (do overtime thresholds reduce by the same percentage as part time?) and paid. And of course it increases complexity in crew planning which in turn costs money.
I would advise you to seek advice from pilot unions close to you, i know ECA has a wide range or contacts if that is close enough.