I’ll just add to what you’ve already found. The Mass and the Eucharist.
While this following scripture from Hebrews, doesn’t mention
• Mortal sin for deliberately missing Mass
Anybody can see what is being described. What they are doing when they meet, and what Day it is that they meet, and that there is disastrous consequences for that person if they deliberately fail to meet on “the Day” once they become knowledgeable of the truth. IOW it’s already a huge sin to not follow this command
Heb 10: (all emphasis mine) https://www.biblegateway.com/passage/?search=heb+10%3A19-31&version=RSVCE
19 Therefore, brethren, since we have confidence to enter the sanctuary by the blood of Jesus, 20 by the new and living way which he opened for us through the curtain, that is, through his flesh, 21 and since we have a great priest over the house of God, 22 let us draw near with a true heart in full assurance of faith, with our hearts sprinkled clean from an evil conscience and our bodies washed with pure water. 23 Let us hold fast the confession of our hope without wavering, for he who promised is faithful; 24 and let us consider how to stir up one another to love and good works, 25 not neglecting to meet together, as is the habit of some, but encouraging one another, and all the more as you see the Day drawing near. 26 For if we sin deliberately after receiving the knowledge of the truth,
• there no longer remains a sacrifice for sins,
• 27 but a fearful prospect of judgment,
• and a fury of fire which will consume the adversaries.
28 A man who has violated the law of Moses dies without mercy at the testimony of two or three witnesses.
• 29 How much worse punishment do you think will be deserved by the man who has spurned the Son of God,
• and profaned the blood of the covenant
• by which he was sanctified, and outraged the Spirit of grace?
30 For we know him who said, “Vengeance is mine, I will repay.” And again, “The Lord will judge his people.”
• 31 It is a fearful thing to fall into the hands of the living God."
to be continued