I had the same dilema several years ago and did some research on the subject. It is not easy to remove the paint. The chemical streapers are expensive and dangerous for the environment. You can use caustic soda for example but then have to rinse it well. Where are all the chemicals going - to your garden, into the drain? Another option is sandblasting but then you might have to seal the bricks as their top layer is likely to be compromised. Another potential problem is that there might be a reason for painting the bricks in the first place - they might not look that good when exposed.

Ah. I see you've met the twin of the previous owner of my place, the king of plastic paints! For the interior paints, I'm looking at using Barrettine's Peelaway. For anything more complex, like your cement render, I tend to look at the SPAB's technical guides. You have my sincerest sympathies, though, as our 1960s box of a flat in London is cement render over thermobrick throughout -- hideous beyond belief. https://www.spab.org.uk/advice/technical-qas/technical-qa-1-dealing-with-inappropriate-cement-renders/

I had my sitting room door dipped many years ago and it was fine. I kept mine natural and just varnished it. I’m far to lazy to be bothered with stripping a door by hand! Here’s a pic

Ask in BandQ or screwfix etc ref the brush for drill and what you want if for and they'll sort you out. Most drills are common in terms of bit connection but maybe take a photo of drill chuck before you head to the shop in case they ask. Generally standard and SDS, but might as well have a photo pre taken ὄDὄD