Is there a way of activating the phone connection (bluetooth) ONLY when there's an activity to sync? (Fenix 7X solar)

I am not sure is the following method still works, have not used it for a long time: maybe you can use a customize a power mode for this, say Max Battery for example. Set that to whatever you want in normal watch mode, just leave the phone connection off. Activate it. Your activities will use the Normal power profile with phone connection on. If you save an activity but stay in that activity, i.e. don't return to the watchface right away, the activity will sync. After sync complete, return to watchface. I use that with a Fenix 6 and it used to work on Fenix 7 as well.

Thanks, I'll give that a go, it sounds like a nice alternative solution.

I don't think it will give you any real benefit in work on battery.
Bluetooth is consuming so minimal amount of power that it is no worth to bother.

Well, in normal watch operation you could be right but for example I have a big problem with it for some time. With Bluetooth off the watch uses less than 3% of battery per day and with bluetooth on - 8-9% per day (fenix 7x ss, beta 18.XX). But actually, some time ago it was not a big problem. I lost about 2 days out of 28.

As a solution, I added the control element "enable/disable connection with the phone" to the quick selection under the light button. I turn it on when i want to sync the acivity.

If it is okay to only sync at home, you could use WiFi. It will only activate when needed.

Weird, I think some issue in BETA.
I'm no longer Fenix user and my watch is not included in BETA tests than I cannot check it.

It must go into receive mode intermittantly to work out if it's available, that consumes power, and these days with complicated protocols, recieve demod can be the biggest power draw overall because it doesn't know if it's needed or not, so it keeps trying, and the demod process is non-trivial.  Thankfully most of the demods can be bought either in chips or I.P. which a manufacturer like garmin can imbed into their chips, but a lot is still going on and that does take a certain amount of energy.

I'm not being picky, I'm unusual, I design radio systems for a living, had a lot of experience developing low power radio, some of the stuff I've worked on makes Bluetooth LE look monsterously power hungry.

Have you already tried whether the "maximize compatibility" mode might solve your problem?
Go to "Connectivity" - Phone - press the light button 5 times:

odd, I don't have that..  I can choose between "adaptive", "1MB" and "2MB", that's all.

It usually also activates a 2nd time about an hour after the activity.