It does look as though you can make an election unless your interest and dividends are substantial! This is because your UK income can be up to 11,784€ in 2024 and you can still make an election. For you, it will be, I think, based on the "income not taxable in Germany is less than the Grundfreibetrag" option, not the "90% of income arising in Germany" option because your UK pension plus investment income looks as though it is probably more than 10% of your income. If your only German income is your pension, I would expect you to be dealt with by FA Neubrandenburg (Rente-im-Ausland) (RIA). They have a lot of info about limited/unlimited liability on their website in both English and German.
Don't forget that not all of your German pension is taxable in Germany because of the change to "nachgelagerte Besteuerung" of the pension back in 2005. (Again, see the RIA website). Depending on the year in which your pension started, and to avoid domestic double taxation, a % is tax-free. This is because, before 2005 your pension contributions were out of taxed income, whereas now, they are out of pre-tax income but the new rules tax the whole pension, subject to that tax-free %. For pensions starting in 2017, 26% of the first full calendar year's pension is tax free. This amount is fixed for life. That tax-free element reduces by 2% for pensions starting in years up to 2020 when it is 20%, and by 1% for pensions starting in 2021 and so on, so those starting in 2024 have a 16% tax-free part. You also get a Werbungskostenpauschal fuer Rentner of 102€. Yes, complicated! That is why I have opted for Amtsveranlagung!
Turning to the calculators, the amounts you use are annual amounts, as the Grundfreibetrag is an annual allowance, and the tax calculators work on annual taxable income. So, with the BMF calculator, add your annual taxable amount of German pension to the Grundfreibetrag and pop that in as your taxable income. In the Progressionsvorbehalt calculator you put your annual taxable amount of German in the Zu versteuerndes Einkommen box (do not add the Grundfrebegtrag this time), and your annual UK pension income plus your interest/dividends, less the Sparer-Pauschbetrag (1,000€ for 2023 onwards and 802€ for earlier years) in the other box.
On the results page you are interested in the line "Einkommensteuer" above the line "Weitere Informationen". If you have 12,000€ taxable amount of German pension (after the tax-free amount and the 102€ are deducted) and, say 2,000€ UK pension and interest etc above the 1,000€ allowance then for 2024 you get: * limited liability tax (no election) 2,699€ * unlimited liability tax (election) 332€ (note - the Progressionsvorbehalt calculator doesn't appear to have been updated yet since the Government recently increased the 2024 Grundfreibetrag by a further 180€, so it gives slightly too high figure - how annoying!) Seems worth making an election in those circumstances but as you say, you may want to take professional advice as this stuff is a tad complicated. Hope this helps.