The following steps are for stained trim/beadboard. Painted trim/beadboard is covered in the last paragraph. In your case, installing the door trim first is step one. To get the base molding to match the door trim, it will need installing second, unless you cheat by planing the base molding thinner to allow it to match the door molding when installed over the beadboard. That will make the base molding thinner and it will be different looking in width from the door molding---but the surfaces will match. That generally is not acceptable to most people. That poses a double problem, however. That problem is how to get the edges of the beadboard to match the edges/bottom of the door and base molding. One way to eliminate this problem is to cut rabbets in the door and base molding. A rabbet is a groove cut into the side of a piece of wood and forms a notch the whole length(or as much as needed for the beadboard on the door molding sides) of the wood. That allows you to fit the edges/bottom of the beadboard in the rabbets and elininates any gaps. The only gaps will then be in the gaps created by the pattern of the beadboard. A colored caulk can be used to fill those if the moldings/beadboard are stained. If you are painting both trim and beadboard, it is much more simple. Install both moldings first. Then install the beadboard. Then caulk the gaps with a paintable caulk, prime, and paint.

I like baseboards larger than door and window casings. On door casings, I like your idea of same design/different width casings. Except maintain consistency like all doors in the old part of the house have the same narrower width casing. You don't want to look down a hallway, for example, and see different width casings on some than on others.

Way too vague to start. Mid century which style? Is painting the reason for keeping the wider trim? Using the same trim in rooms where the trim continues is necessary, but in bedrooms/bathrooms/contained rooms, trim can be different since the trim is contained in that room. What look do you want? Simple? Elaborate? In between? That has a huge impact on the style of trim and profile.

Would you mind sharing why you have a paint crush on the Break-through by PPG?? ProClassic satin (Extra White) is actually gorgeous and I love it. But BM Advance and SW ProClassic both take hours in between coats. My son, Jake, is doing the painting for me while he's home from college for the summer and he prefers to spray early morning and let it dry overnight. He's sanding between coats of primer and first coat of finish because the cabinets are old with rough spots. Sanding between coats helps a lot so it's totally worth the extra work. ProClassic takes a good 14 days to cure out so you can get a feel (literally) for the final finish and color; when it's fully "cooked" I like it a lot. But it's a slow process to get there with both brands (BM and SW) and the first day I didn't care for the chalky finish but that quickly changed as the paint cured. Break-Through is different you can sand/recoat in two hours and cures to final appearance within maybe 2-3 days or so. All of them sprayed, rolled, leveled nicely for us. We are not using a professional grade sprayer, just something we picked up at Lowe's. I have nothing bad to say about the paint BM or SW at all. Break-Through just dries and cures faster. ProClassic satin the is "prettiest" IMO but I'm super critical and it's not enough of a difference to sacrifice the chunk of time it requires to get to that pretty final finish - I just haven't had the extra time to spare with my recent projects so Break-Through has been a good solution. When I spec color for homeowners most painters around here spray lacquer with pro grade sprayers and it's a whole, huge production with masking and paper and tape.... and it stinks. We don't have the bandwidth or space to go that route. With Break-Through you have to wipe up any spills or splatter immediately because once it dries, it's not coming off - kind of wicked and unforgiving in that way.