Options under EC261 in event of partial cancellation on a multicity itinerary

I have a multi city booking on a single PNR, as follows:

Segment 1 / Leg 1 BRS - AMS, Saturday (operated by KLM)
Segment 2 / Leg 2 AMS - TLL, Tuesday (operated by Air Baltic)
Segment 3 TLL-BRS Friday: comprised of leg 3 TLL - AMS (operated by Air Baltic) connecting to leg 4 AMS - BRS (operated by KLM).

Booking was with KLM, Air Baltic is a codeshare partner so those flights also carry a KL flight number.

Estonia only permits flights from airports in countries where the recent COVID morbidity is less than 25 cases per 100,000, reviewed each Friday. Unfortunately the most recent figures for the Netherlands have exceeded that since I booked. Presumably as a result, Air Baltic has been cancelling their AMS-TLL flights on a rolling basis since Friday (it looks like they wait until one or two days before departure to pull each flight, however, rather than removing a week's worth immediately).

There are still a few weeks for the numbers to improve sufficiently that my itinerary will remain valid, but I am unsure how to parse the usual EC261 options of reimbursement, re-routing or return if there is no such improvement and hence a partial cancellation does occur. Specifically:

If the Air Baltic flights are cancelled, am I entitled to adjust my AMS-BRS leg to some other date (ideally the Tuesday originally intended for AMS-TLL), since I would still wish to visit the Netherlands Sat-Mon?
If I am instead expected to stick to the original AMS-BRS leg on the Friday, is there any duty of care for accommodation in the Netherlands Tuesday through Friday? Whilst my bookings in Tallinn are refundable, last minute Dutch hotels are more expensive than pre-booked Estonian ones.
How would reimbursement be calculated if only the middle two legs are cancelled?
Do the answers to any of the above depend on whether I have commenced travel (i.e. on whether Air Baltic cancels the flight immediately after figures are released on Friday afternoon, versus some time after I've arrived in the Netherlands on Saturday morning)?

Note that I am only interested in my options regarding reimbursement/return; this is not a question about seeking additional compensation. I am also aware of the awkward possibility of a COVID figure between 15 and 25, whereby the flights are allowed to operate but I would have to quarantine in Estonia for two weeks (in which case it then becomes me who would wish to cancel some legs, not Air Baltic).