Timing of bike fit

I'm having the same question as the TS. I'm afraid I can't give enough feedback during the fitting, because I'm not used to ride that kind of bike.

I'm probably going to buy a decent but not too expensive bike, start riding with some friends. And with a bit of trail & error, DIY adjustments I can start getting used to the race bike geometry.

Assuming that after a couple of months, I will have a better idea on how to sit on a road bike, so the fitter only has to do some finetuning. That he doesn't have to explain me how to use drop bars

My bike feels too small on the way to work and too big on the way home.
I guess that means it is about right for a lunchtime ride

It has evolved a lot since I got it though: spacers under stem have dwindled; saddle raised and set back further; longer stem; narrower bars.

In other words, your body will change and what fits today may not fit in 6 months or a year. Getting a comprehensive bike fit right now would be a waste of cash imho. Go for something that is a ballpark fit based on height/inseam etc. and then adapt it and your body as you get the miles under your belt.

At the end of last summer I was considering a longer stem because I felt hunched up on my CX. Now, after a not-very-strenuous autumn and some festive weight gain... I can hardly reach the bars. Fittings should make a big difference to your effectiveness but they don't change how long your legs and arms are. I'd buy a bike that's roughly the right size first, ride for a few months (tinker with your position of course) and then have the fit.

I'm not so sure. Of course the bike needs to fit, but the body changes so much as you adapt to riding a road bike that you may then need a second fitting. My initial fitting was not the same as my fitting a year later, but that's not to say that the first guy was incompetent, it's more the fact that I had changed in the meantime. You get more flexible, you find that you get aches here but not there etc. Your neck and back might ache if you're transitioning from an MTB/hybrid to a road bike, since the position is more stretched out than on other types of bike, but that isn't necessarily a reason to change your position - to a certain extent it's just something you have to adjust to, strengthening your muscles accordingly.

I'd buy from a reputable local bike shop. Either they will get your position more-or-less right, and you can put in a few months of riding before getting your bike fit done, OR, if it's a really good place that does proper bike fits, then you can talk to them to try and come to an arrangement. A good fitting will often be included in the purchase price, which is great if you're already in the habit of putting the miles in on a dropped-bar bike, but you could ask them about doing the bike-fitting initially but then re-checking your position when you take it in for the first service (assuming you get to put in a decent amount of time on it). Actually, I suppose you could try having the same conversation with whichever bike fitter you find - that way you could cover all the bases.

Get the bike fit done first, this will fix your contact points to the right areas.
Then when you shop for a bike, you know what frame/stem/crank/seatpost combinations will work for you.

As above... fit first, but a frame that fits you, rather than trying to retrofit you to a particular purchase.

personally I'd go for the fit first. that way you know exactly the size and set up
of your new bike before you even get out of the shop ... bonus is that you'll not
have any aches or pains, well apart from good ones, for the very beginning