they need hundreds of thousand of soldiers to enable a credible defence

And arm them with what? Modern wars are extremely expensive and Finland may have conscription but the country's military spending as a portion of GDP is lower than that of many countries with professional armies. Conscription makes sense in Israel. But they get about as much in American arms donations as Finland spend on its entire military.

The whole setup looks Finland looks like the strategy is to copy Stalin's tactic in the winter war: sacrificing soliders en masse.

Sure, without Nato and the EU there could be the scenario we now see in Ukraine: Hundreds of billions of dollars in foreign aid paying for weapons. But since Finland is in Nato it would get actual armies coming to its aid.

And I'm not even talking about the indirect costs of conscription. Diminishing your workforce by a percent also dimishes your GDP by (roughly) a percent. With that money you could create and maintain a serious nuclear arsenal, including second strike capabilities.