Gasket Masters replaced my HG - next steps?

This is the next (and hopefully last!) chapter in the story from my thread started two weeks ago:
Blown head gasket, should I swap engines or... ? | PriusChat

In short, I had a blown head gasket and too-high quotes from the only dealership in the area and an independent shop. I posted here and thanks to several of you, decided to drive 6 hours to Manteca, CA, spend the night in a hotel, have them replace it the next day, and drive home that evening. So I spent most of two days and around $2300 all in, drove around 700 miles round trip, and now I have a new head gasket on an old-ish car. I hope to keep the car at least 5 years and maybe another 100k, so what should I do to increase my odds?

The car already had the brake booster replaced about 6 months ago (8 months after the TSB expired that would have made it free). About 3 months ago when the P0301 misfire first popped up, I did lots of maintenance work: cleaned the whole EGR system and the intake manifold, cleaned fuel injectors, swapped ignition coils, replaced spark plugs, changed coolant, replaced the PCV valve and added an oil catch can, etc. We've had it about 4 years and 40k miles, and I've noticed a very small drop on the oil dipstick between changing it every 8-10k or so (before the HG problem), barely enough to register. Also, both the dealership and the independent shop said the timing cover gasket was leaking a tiny bit, so I have reason to believe it's not burning oil.

To prolong the HG life, it seems like I should do the following every 50k miles: clean the EGR system and IM (especially the EGR passages), change coolant and spark plugs. Plus change oil every 5k instead of 10k, and of course empty the OCC when changing oil. Are those necessary? Am I missing anything? Also, I have a plausible theory (thanks @Mendel Leisk for spreading the word) for why the EGR system design leads to localized overheating in cylinder 1 and 2 and causes the HG failure in that spot, but I don't understand why more-frequent oil changes and coolant changes help prevent HG failure? What happens to the oil and coolant with more miles that would affect the HG?

Also, for anyone curious, here's my experience with Gasket Masters:
I called (the Manteca location directly, not the 800 number) twice before driving there, first to get questions answered and make a plan, then to let them know I was coming and ask details about the work - they use Felpro HG and all others Toyota, new head bolts, they change valve stem seals and lap valves, measure cylinder height to test for bent rods, measure flatness on head and block, etc. Only $2150 including tax!

I arrived after 10pm, spent the night in the cheap hotel right across the street, and got the car there just after 8am when they opened. Manteca is a working class town and Gasket Masters has a tiny space - just one bay and a tiny office in a strip of auto and machine shops, no signs. Manuel put my phone number and name in the notes app on his phone and called me to be sure the number was right, took the key and said they'd have it done by the end of the day - no paperwork, no systems, no pretense. They had just started a car that had been there the day before, and after a few hours I checked out of the hotel and walked over to put things in the car. They hadn't started my car yet, Manuel was on the phone when I walked over, and just pulled my key out of his pocket so I could put stuff in - like a friend instead of a customer. They were finishing the other car, I walked about 20 minutes to the public library with my laptop. They hadn't called by 2pm when I had lunch, so I walked back and they had the HG on already, and were squirting goo for the FIPG on the timing chain cover. He did it so fast I almost missed it, fingers shaped carefully and following the gun to shape and smooth the bead. Then he put it on without help, a moment to line it up and then a single smooth motion, setting bolts in place with one hand while holding the cover with the other, then zipping them down with the electric ratchet - he was torquing them less than 2 minutes after squirting the FIPG goo on the cover. I was there asking questions around 10 minutes, and the 3 guys there were all answering as they worked, very chill and joking around. It was obvious that they've done hundreds of these and are super familiar with the Prius engine. It reminded me of when people ride with me and see me do a difficult parallel parking job in one quick move, and I have to remind them that I used to be a long haul truck driver, so I can chitchat while doing difficult driving maneuvers, and these guys can chitchat while doing this one particular car repair.

At least I hope that's true! I left just after 4pm, having paid on their Square phone app and carrying a duplicate of the one-page written job as my receipt and warranty. They don't have a computer system or office phone or anything - it's literally just three guys answering their smartphone and getting people in and out so fast they don't seem to need any system more than Manuel putting my key in his pocket. I'm keeping a close eye on the coolant level for the next several months, and will change oil in the next few thousand miles, and will be kind of waiting for the other shoe to drop for a while. For now, I'm just glad to have reasons to trust my car again!