How to connect routers to the core network?

Question

Computer networks

Greetings.
There are three Cisco ISR4331 routers in stock with increased memory with SEC licenses. One port of each is included in the switch: it combines them into one L2 segment. The rest of the ports, routers look at other people's networks, BGP, that's all. Between OSPFv3 routers and iBGP fully meshed logical topology without RR.
The switch combines access switches at the levels below, behind which there are consumers (servers in the main). In this scheme, all routers on the network side are equivalent and are GW for access segments (HSRP). In this scheme, the point of failure is the central switch and it cannot be removed from the scheme without downtime.
There was a question about altering the core network to a more fault-tolerant scheme.
First: how to properly combine border routers with each other? Insert switch-cards into them and combine the ring into L2? There is a risk of loops, but RSTP should help avoid them. What are the disadvantages of this solution? Insert L3 cards and merge them with p2p junctions? So far, three routers, each has three expensive L3 ports, and then progression (while maintaining the physical full mesh). Further, it is planned to transfer access from routers to switches (Cat3850), which so far work as L2. Switch some ports on switches to L3 mode (or use SVI), drive OSPF and default there, make a dead end area. Switches (two) will work in conjunction with HSRP, then connect with uplinks to routers. What is the best way to connect two switches to three routers in this case to ensure fault tolerance?

Answer 1

I will advise only on the very first part of the question and only as a theoretician. Cisco stands up for L3 wherever possible (and drags it all the way to access switches). L3 solves problems with loops, reduces network convergence time in case of equipment failure.

Answer 2

If it is possible to buy cards, then I would buy and connect them with switches (which act as HSRP), if the cards have 2 ports, then from two routers, 2 links for each switch, you can make L3 etherchannel from the nickname, and from the third router, one link for each switch and also L3 p2p. If there is no way to buy cards and there are free ports on the switches, then you can do the same, just raise the L2 etherchannels and transfer the uplinks of other networks from the router to the switches and on the router on the subinterfaces, raise the routing with BGP providers