Having dealt with lots of tradespeople over the years, the only ones who have insisted on big sums up front, then pestered for interim payments, have been wastes of space. YMMV and all that, but this has been my experience.
There’s nowt wrong with asking, or indeed, getting interim payments during a long job, as long as the work invoiced has been completed.
There’s one hell of a lot of trust involved, for both sides, when you get tradesmen into your home. Expecting someone to stump up for materials and then work unpaid for two or three weeks, or months, is unrealistic, but a few days really isn’t.