The last daytime always stretches, in my experience. But I think I do naps in a very different way to you op.
Crucially, "daytime" means my daytime (ie 7am to 11pm, or whenever I go to bed) until baby starts to independantly stay asleep through the evening, when I would start introducing the earlier evening bedtime.
In my experience this development to stretch the last nap to become part of night sleep hapiens around 4-8 months, so the timing is about right for your LO.
But as I say, you are looking at napping in a very different way to me. Until naps lengthen, while baby is still young, I would be much more organic and be led by baby.
For example from 7am, I could fivide the day into 2 hour block of time. In every 2h block there would be a feed and a nap. Say:
7am wake
Feed, play, activity
8.30 sleep
9am wake
Feed, play, activity
10.30 sleep
11am wake
Feed, play, activity
12.30 sleep
... And so on. You get the idea. Until:
5pm wake
Feed, play, activity
6.30pm sleep
7pm wake
Feed play and "do bedtime"
(Bath, night clothes, massage etc)
8.30 sleep
(This continues to be a nap though. Just like the rest of the daytime naps)
So this last nap. Maybe it will last 30 minutes like all the others. So if waking at 9pm it would be quite normal but the only difference would be no playtime. Just feed, straight back to sleep.
But without me doing anything, the nap will start lasting longer. So it might be go to sleep at 8.30pm (for a nap, but in PJs) and not wake for the next feed until 10pm, or 10.30pm. Once it gets to the stage that I wake baby when i want to go to bed, rather than the other way around, that means that "bedtime" (meaning going to sleep and staying asleep in the early evening) starts, so baby goes to sleep upstairs. I've usually got to this point around 5-6 months old.
The nap at the end of the day extending is therefore normal development, in my world.
Then as sleep matures, and naps get longer, awake time between daytime naps extends and the last nap of the afternoon isn't so much "dropped", it's just that extending naps times and a wake times means there isnt time in the day for it.
My feeling is that you are being overtly structured before baby is ready. The time for "Mummy Routine Queen" will come. It's usually when naps extend and consolidate though.